SMTP, IMAP, POP3 and MIME: understanding email protocols

Introduction and big picture

Email is a fairly old school technology meant to replicate postal service digitally. To understand how email works, we must know about network protocols that specify sets of data formats and data exchange rules for exchanging messages over the network. In the modern internet email sending part is conceptually (and sometimes technically) separate from receiving part. Email sending aspects are formalised in RFCs that describe SMTP protocol, whereas reception part can be done either via IMAP or POP3 protocol. The conceptual difference between IMAP and POP3 is that IMAP is meant to provide a way to browse mailboxes via network connection whereas POP3 is meant to download received messages into user-controlled computer and let them read it offline without any persistent connection to the server. Furthermore, MIME data format is used for encoding contents and inner structure of the messages that are transferred.

Broadly speaking, email system is federated and consists of the following parts:

Mail Transfer Agent (MTA) is a program that implements SMTP protocol to transport messages between hosts (e.g. Sendmail, qmail, Postfix).
Mail User Agent (MUA) is the email client application (e.g. mutt, Mail.app, Outlook, Thunderbird).
There can also be a Mail Delivery Agent (MDA) - an intermediate piece of software that bridges the gap between MTA and MUA on the receiving side (e.g. procmail). This can be done for spam filtering purposes and to manage the email message persistence on Unix/Linux systems.
Lastly, there can also be a Mail Submission Agent (MSA) that is equivalent of MDA, but for sending the email.

The general big-picture flow of email message is the following:

A sender creates the email message and sends it from MUA to MTA managed by their email provider (possibly via MSA). SMTP protocol is being used here.
Message traverses one or more MTAs. Again, this is done via SMTP protocol.
Message reaches the final MTA belonging to the email provider of the recipient and is stored there.
Recipient uses MUA on their end to retrieve the message (possibly via MDA). Either IMAP or POP3 protocol can be used here.

It should be noted that email provider specific REST API can be used between MUA and MTA (possibly through a client library). For examples of such APIs, see:

This would replace SMTP, IMAP and POP3 protocols for communications between client and server.

SMTP: Simple Mail Transfer Protocol

SMTP was originally formalised back in 1981 in RFC 788. Since then, the protocol specification has been extended across various RFCs to keep it up to data with the changing needs of growing internet. At the time of writing, RFC 5321 is the final version of core SMTP spec. SMTP is a text-based protocol with two kinds of messages: commands and responses.

Let us use smtplib module from the vanilla Python installation to explore the SMTP debugging output. We want to see the protocol flow.

$ python3
Python 3.10.8 (main, Oct 13 2022, 09:48:40) [Clang 14.0.0 (clang-1400.0.29.102)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import smtplib
>>> smtp_conn = smtplib.SMTP("smtp.rambler.ru", port=587)
>>> smtp_conn.set_debuglevel(1)
>>> smtp_conn.starttls()
send: 'ehlo [[REDACTED]]\r\n'
reply: b'250-mail.rambler.ru\r\n'
reply: b'250-SIZE 104857600\r\n'
reply: b'250-ENHANCEDSTATUSCODES\r\n'
reply: b'250-8BITMIME\r\n'
reply: b'250-AUTH PLAIN LOGIN\r\n'
reply: b'250 STARTTLS\r\n'
reply: retcode (250); Msg: b'mail.rambler.ru\nSIZE 104857600\nENHANCEDSTATUSCODES\n8BITMIME\nAUTH PLAIN LOGIN\nSTARTTLS'
send: 'STARTTLS\r\n'
reply: b'220 2.0.0 Start TLS\r\n'
reply: retcode (220); Msg: b'2.0.0 Start TLS'
(220, b'2.0.0 Start TLS')

We established TCP connection to SMTP server and sent EHLO command with our IP address to initiate the conversation. The server responded with status code 250 (indicating success) and a list of SMTP service extensions that server is supporting. To make the connection encrypted, we also issued a STARTTLS command. To this command server replied with status code 220, which means that it ready to serve the client.

Now we need to login to the server with account credentials.

>>> username = "[REDACTED]"
>>> password = "[REDACTED]"
>>> smtp_conn.login(username, password)
send: 'ehlo [202.3.218.138]\r\n'
reply: b'250-mail.rambler.ru\r\n'
reply: b'250-SIZE 104857600\r\n'
reply: b'250-ENHANCEDSTATUSCODES\r\n'
reply: b'250-8BITMIME\r\n'
reply: b'250 AUTH PLAIN LOGIN\r\n'
reply: retcode (250); Msg: b'mail.rambler.ru\nSIZE 104857600\nENHANCEDSTATUSCODES\n8BITMIME\nAUTH PLAIN LOGIN'
send: 'AUTH PLAIN [REDACTED]\r\n'
reply: b'235 2.0.0 OK\r\n'
reply: retcode (235); Msg: b'2.0.0 OK'
(235, b'2.0.0 OK')

Calling login() method repeated the EHLO message over the now-encrypted channel and sent AUTH message that deals with “PLAIN Simple Authentication and Security Layer (SASL) Mechanism” described in RFC 4616. The redacted part is Base64 string from the following sequence:

Null byte (0x00).
Email account username.
Null byte.
Email account password.

We got response from the server saying that all is good with the login step. Now we can send some test message.

>>> to_addr = "[email protected]"
>>> smtp_conn.sendmail(username, to_addr, "Test!")
send: 'mail FROM:<[REDACTED]> size=5\r\n'
reply: b'250 2.1.0 Ok\r\n'
reply: retcode (250); Msg: b'2.1.0 Ok'
send: 'rcpt TO:<[email protected]>\r\n'
reply: b'250 2.1.5 Ok\r\n'
reply: retcode (250); Msg: b'2.1.5 Ok'
send: 'data\r\n'
reply: b'354 End data with <CR><LF>.<CR><LF>\r\n'
reply: retcode (354); Msg: b'End data with <CR><LF>.<CR><LF>'
data: (354, b'End data with <CR><LF>.<CR><LF>')
send: b'Test!\r\n.\r\n'
reply: b'250 2.0.0 Ok: queued as CB14F2642E52\r\n'
reply: retcode (250); Msg: b'2.0.0 Ok: queued as CB14F2642E52'
data: (250, b'2.0.0 Ok: queued as CB14F2642E52')
{}

There a sequence of three steps here:

We provide the email address of the sender which will correspond to email account username in most cases via MAIL FROM command.
We provide recipient address via RCPT TO command.
We send DATA command, contents of the message and message end sequence. In this case the message is a very simple ASCII string, but in real world this is typically a MIME payload that will be discussed later.

All step succeeded and a message was enqueued for further transfer. To finalise the connection in a clean way we would send a QUIT command.

We only covered the happy path here. RFC 5321 Appendix D lists more examples.

Futhermore, modern SMTP implementation deal with additional considerations, such as spam filtering and policy enforcement.

Standard TCP port of plaintext SMTP is 25. SMTP-over-TLS typically uses port 587.

One might ask: given an email address, how does MTA know the IP address of the next MTA to pass the message to? This is typically found through DNS MX records that point to hostname of the mail server servicing the given domain name.

IMAP: Internet Message Access Protocol

IMAPv4rev2 is the current version of IMAP protocol, specified in RFC 9501. This is fairly new standard that is not always implemented to the letter. However, IMAP protocol has been around for long time and is widely supported in the world of network software. Just like SMTP, IMAP is text-based, line-oriented protocol.

Let us receive an email with imaplib Python module and go through IMAP protocol flow. We will enable debug output in Python REPL and reproduce the steps that email client would do when receiving messages.

Like with SMTP protocol, we first need to establish the connection and login to an account.

>>> import imaplib
>>> imap_conn = imaplib.IMAP4_SSL("imap.rambler.ru", port=993)
>>> imap_conn.debug = 5
>>> imap_conn.login(username, password)
  33:33.96 > b'OGCE1 LOGIN [REDACTED] "[REDACTED]"'
  33:34.54 < b'OGCE1 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA] Logged in'
  33:34.54 	matched b'(?P<tag>OGCE\\d+) (?P<type>[A-Z]+) (?P<data>.*)' => (b'OGCE1', b'OK', b'[CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA] Logged in')
  33:34.54 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'CAPABILITY', b' IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA', b'IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA')
  33:34.54 untagged_responses[CAPABILITY] 0 += ["b'IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA'"]
('OK', [b'[CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY SPECIAL-USE QUOTA] Logged in'])

Unlike with SMTP connection, we establish TLS connection to server first and start sending IMAP commands in the encrypted channel. IMAP protocol does not have any equivalent of STARTTLS message to upgrade plaintext connection to an encrypted one.

LOGIN command was sent with username and password. Server responded with status OK and a list of supported capabilities.

Now we need to select a mailbox on the server. If we provide no mailbox name the Python IMAP client will switch to inbox by default.

>>> imap_conn.select()
  34:03.21 > b'OGCE2 SELECT INBOX'
  34:03.72 < b'* FLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft)'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'FLAGS', b' (\\Answered \\Flagged \\Deleted \\Seen \\Draft)', b'(\\Answered \\Flagged \\Deleted \\Seen \\Draft)')
  34:03.72 untagged_responses[FLAGS] 0 += ["b'(\\Answered \\Flagged \\Deleted \\Seen \\Draft)'"]
  34:03.72 < b'* OK [PERMANENTFLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)] Flags permitted.'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'OK', b' [PERMANENTFLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)] Flags permitted.', b'[PERMANENTFLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)] Flags permitted.')
  34:03.72 untagged_responses[OK] 0 += ["b'[PERMANENTFLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)] Flags permitted.'"]
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'PERMANENTFLAGS', b' (\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)', b'(\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)')
  34:03.72 untagged_responses[PERMANENTFLAGS] 0 += ["b'(\\Answered \\Flagged \\Deleted \\Seen \\Draft \\*)'"]
  34:03.72 < b'* 10 EXISTS'
  34:03.72 	matched b'\\* (?P<data>\\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?' => (b'10', b'EXISTS', None, None)
  34:03.72 untagged_responses[EXISTS] 0 += ["b'10'"]
  34:03.72 < b'* 4 RECENT'
  34:03.72 	matched b'\\* (?P<data>\\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?' => (b'4', b'RECENT', None, None)
  34:03.72 untagged_responses[RECENT] 0 += ["b'4'"]
  34:03.72 < b'* OK [UNSEEN 7] First unseen.'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'OK', b' [UNSEEN 7] First unseen.', b'[UNSEEN 7] First unseen.')
  34:03.72 untagged_responses[OK] 1 += ["b'[UNSEEN 7] First unseen.'"]
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'UNSEEN', b' 7', b'7')
  34:03.72 untagged_responses[UNSEEN] 0 += ["b'7'"]
  34:03.72 < b'* OK [UIDVALIDITY 1644072125] UIDs valid'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'OK', b' [UIDVALIDITY 1644072125] UIDs valid', b'[UIDVALIDITY 1644072125] UIDs valid')
  34:03.72 untagged_responses[OK] 2 += ["b'[UIDVALIDITY 1644072125] UIDs valid'"]
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'UIDVALIDITY', b' 1644072125', b'1644072125')
  34:03.72 untagged_responses[UIDVALIDITY] 0 += ["b'1644072125'"]
  34:03.72 < b'* OK [UIDNEXT 11] Predicted next UID'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'OK', b' [UIDNEXT 11] Predicted next UID', b'[UIDNEXT 11] Predicted next UID')
  34:03.72 untagged_responses[OK] 3 += ["b'[UIDNEXT 11] Predicted next UID'"]
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'UIDNEXT', b' 11', b'11')
  34:03.72 untagged_responses[UIDNEXT] 0 += ["b'11'"]
  34:03.72 < b'* OK [HIGHESTMODSEQ 17] Highest'
  34:03.72 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'OK', b' [HIGHESTMODSEQ 17] Highest', b'[HIGHESTMODSEQ 17] Highest')
  34:03.72 untagged_responses[OK] 4 += ["b'[HIGHESTMODSEQ 17] Highest'"]
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'HIGHESTMODSEQ', b' 17', b'17')
  34:03.72 untagged_responses[HIGHESTMODSEQ] 0 += ["b'17'"]
  34:03.72 < b'OGCE2 OK [READ-WRITE] Select completed (0.009 + 0.000 + 0.008 secs).'
  34:03.72 	matched b'(?P<tag>OGCE\\d+) (?P<type>[A-Z]+) (?P<data>.*)' => (b'OGCE2', b'OK', b'[READ-WRITE] Select completed (0.009 + 0.000 + 0.008 secs).')
  34:03.72 	matched b'\\[(?P<type>[A-Z-]+)( (?P<data>.*))?\\]' => (b'READ-WRITE', None, None)
  34:03.72 untagged_responses[READ-WRITE] 0 += ["b''"]
('OK', [b'10'])

You may notice that first command message was prefixed by OGCE1 and next one by OGCE2. This is IMAP message tagging in action. They are meant to associate some of the reply messages with a command message, as is seen in one of the lines that client got in reponse to SELECT message.

We got several lines in response that mostly deal with message UIDs - 32-bit numbers that uniquely identify messages within the email account.

* FLAGS (\\Answered \\Flagged \\Deleted \\Seen \\Draft) - a list of flags that can be assigned to messages (answered, seen, deleted, etc.)
* 10 EXISTS - number of messages in the mailbox (10).
* 4 RECENT - number of messages with “recent” flag set (4) that recently arrived to mailbox.
* OK [UNSEEN 7] - UID of the first unseen message.
* OK [UIDNEXT 11] - next UID that would be assigned on newly arrived message (predicted value).

Now we can search within selected mailbox for messages matching some predicate (e.g. sender address being equal to known value). For the purpose of this example, we will retrieve a list of UIDs of all messages in the inbox.

>>> imap_conn.search(None, 'ALL')
  34:42.42 > b'OGCE3 SEARCH ALL'
  34:42.73 < b'* SEARCH 1 2 3 4 5 6 7 8 9 10'
  34:42.73 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'SEARCH', b' 1 2 3 4 5 6 7 8 9 10', b'1 2 3 4 5 6 7 8 9 10')
  34:42.73 untagged_responses[SEARCH] 0 += ["b'1 2 3 4 5 6 7 8 9 10'"]
  34:42.73 < b'OGCE3 OK Search completed (0.001 + 0.000 secs).'
  34:42.73 	matched b'(?P<tag>OGCE\\d+) (?P<type>[A-Z]+) (?P<data>.*)' => (b'OGCE3', b'OK', b'Search completed (0.001 + 0.000 secs).')
  34:42.73 untagged_responses[SEARCH] => [b'1 2 3 4 5 6 7 8 9 10']
('OK', [b'1 2 3 4 5 6 7 8 9 10'])

We got a list of UIDs from 1 to 10. To get a single message, we can send a FETCH command (response removed for the sake of brevity):

>>> imap_conn.fetch(b'10', '(RFC822)')
  35:08.76 > b'OGCE5 FETCH 10 (RFC822)'
  35:09.15 < b'* 10 FETCH (FLAGS (\\Seen \\Recent) RFC822 {2940}'
  35:09.15 	matched b'\\* (?P<data>\\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?' => (b'10', b'FETCH', b' (FLAGS (\\Seen \\Recent) RFC822 {2940}', b'(FLAGS (\\Seen \\Recent) RFC822 {2940}')
  35:09.15 	matched b'.*{(?P<size>\\d+)}$' => (b'2940',)

In the response we get a MIME payload with all the headers.

Finally, we close the connection and logout:

>>> imap_conn.close()
  36:17.68 > b'OGCE7 CLOSE'
  36:17.96 < b'OGCE7 OK Close completed (0.001 + 0.000 secs).'
  36:17.96 	matched b'(?P<tag>OGCE\\d+) (?P<type>[A-Z]+) (?P<data>.*)' => (b'OGCE7', b'OK', b'Close completed (0.001 + 0.000 secs).')
('OK', [b'Close completed (0.001 + 0.000 secs).'])
>>> imap_conn.logout()
  36:22.24 > b'OGCE8 LOGOUT'
  36:22.57 < b'* BYE Logging out'
  36:22.57 	matched b'\\* (?P<type>[A-Z-]+)( (?P<data>.*))?' => (b'BYE', b' Logging out', b'Logging out')
  36:22.57 untagged_responses[BYE] 0 += ["b'Logging out'"]
  36:22.57 BYE response: b'Logging out'
('BYE', [b'Logging out'])

POP3: Post Office Protocol version 3

POP3 is a network protocol that enables end users to download email messages for offline reading and archiving. Python supports it via poplib module. Like SMTP, POP3 supports both using TLS from start of connection and upgrading the plaintext connection before login step to POP3-over-TLS.

Overall POP3 is much simpler protocol than IMAP. Listing messages and retrieving them one at a time is fairly simple:

>>> import poplib
>>> pop_conn = poplib.POP3("pop3.rambler.ru")
>>> pop_conn.user(username)
b'+OK'
>>> pop_conn.pass_(password)
b'+OK Logged in.'
>>> pop_conn.list()
(b'+OK 10 messages:', [b'1 18561', b'2 18978', b'3 58337', b'4 47221', b'5 48119', b'6 47562', b'7 66608', b'8 71147', b'9 32400', b'10 2940'], 90)
>>> pop_conn.retr(1)

To gracefully close the connection, we send a QUIT command:

>>> pop_conn.quit()
b'+OK Logging out.'

For a specification of POP3, see RFC 1939.

MIME: Multipurpose Internet Mail Extension

Mail client apps often have the feature to view the message in it’s raw form. One would notice that email messages consist of headers and payload, not unlike HTTP requests and and responses.

A very simple MIME message would be something like:

Content-Type: text/plain
MIME-Version: 1.0
To: <[email protected]>
From: <[email protected]>
Subject: Sample MIME Message

This is a simple MIME Message!

Headers being used here are pretty much self-explanatory. This kind of message is a single part one - there is a single type of payload of given content type. For plain text messages message contents are transferred in their original form. If the payload was of some binary format (e.g. PDF document or image file) it would be represented in Base64 form. Some of the popular MIME content types are:

text/plain - just a plain text
text/html - HTML document
image/png - PNG image
application/pdf - PDF document

But nowadays many, if not most, email messages are more complex than this. MIME messages can also be multipart - consisting of several segments.

RFC 1341 provides the following example of two-part MIME message:

From: Nathaniel Borenstein <[email protected]> 
To:  Ned Freed <[email protected]> 
Subject: Sample message 
MIME-Version: 1.0 
Content-type: multipart/mixed; boundary="simple 
boundary" 

This is the preamble.  It is to be ignored, though it 
is a handy place for mail composers to include an 
explanatory note to non-MIME compliant readers. 
--simple boundary 

This is implicitly typed plain ASCII text. 
It does NOT end with a linebreak. 
--simple boundary 
Content-type: text/plain; charset=us-ascii 

This is explicitly typed plain ASCII text. 
It DOES end with a linebreak. 

--simple boundary-- 
This is the epilogue.  It is also to be ignored.

Multipart messages can also be mixed and alternative. Mixed messages (content type multipart/mixed) are multipart messages with parts of different content types that are independent and meant to be displayed serially (one after another). Alternative messages (content type multipart/alternative contain at least two parts that are equivalent representation of the same information (e.g. the same message could be in plain text and HTML). MIME messages are powerful enough to form a tree structure with message as a root node and parts as descendant nodes. That way, parts of message could have child parts with files or previous messages as attachments.

Various modules in Python email package deal with parsing and generating the MIME messages.