It appears that the SMTP EmailPolicy object does not correctly set max_line_length.

RFC 2045 (https://www.ietf.org/rfc/rfc2045.txt) requires a max_line_length of 76 characters, while email._policybase.Policy sets it to 78 (which typically is correct).

This causes email attachments to be truncated/corrupted.

While the workaround is quite trivial, debugging the root cause was not as easy. Thus I think this should be fixed in at the std lib level, or at the very least mentioned in the docs.

This was tested on Python 3.5.2. I did not test on 3.7, but I did check the source code and it appears that the bug still exists.



To reproduce:

    import smtplib
    from email.message import EmailMessage
    from email.policy import SMTP

    msg = EmailMessage()
    msg['Subject'] = "Test message"
    msg['To'] = "to@valid_address.com"
    msg['From'] = "from@valid_address.com"
    content = "hello world, I should have an attachment"
    msg.set_content(content)

    maintype = 'image'
    subtype = 'png'
    test_file = './aaa.png'

    with open(test_file, 'rb') as fp:
        data = fp.read()
    msg.add_attachment(data,
                       maintype=maintype,
                       subtype=subtype,
                       filename='aaa.png')

    with smtplib.SMTP_SSL('smtp.address.com', port=465) as s:
        s.ehlo()
        user = 'smtp_user@valid_address.com'
        pw = 'smtp_password'
        s.login(user, pw)
        s.send_message(msg)



Check the raw email message received. You'll see that lines have a length of 78 and are padded with "==" which makes the total length 80 chars:

    ...
    MIME-Version: 1.0
    Content-Type: multipart/mixed; boundary="===============1585876390557366757=="

    --===============1585876390557366757==
    Content-Type: text/plain; charset="utf-8"
    Content-Transfer-Encoding: 7bit

    hello world, I should have an attachment

    --===============1585876390557366757==
    Content-Type: image/png
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="aaa.png"
    MIME-Version: 1.0

    iVBORw0KGgoAAAANSUhEUgAABE0AAALFCAYAAAA2rB2uAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjw==
    C/xhBQAAACBjSFJNAAB6JgAAgIQAAPoAAACA6AAAdTAAAOpgAAA6mAAAF3CculE8AAD/jklEQVR4Xg==
    7P1rsHVJWecBvl4RFFAuIpcCEYQuLkUDBcgBoQsULLnJXaoaEApe5FLSTVFUC4JACQ1NiQdsQoFBAQ==
    iW4aQSWEt4cQwwB6xqC+CPYHqX5jQno+jHyYCCMmZsKJmYjZs3/rnOe8uXNnrpW5LrlzrfVfVSfOeQ==
    986Vl/+T678y//nkk9+3OXdu+78uISAEhIAQEAJCQAgIASEgBISAEBACQkAICIEdBBBN9CMM1AfUBw==
    ...



Next, modify the code with:

    from email.policy import SMTP
    smtp_pol = SMTP.clone(max_line_length=76)
    msg = EmailMessage(policy=smtp_pol)
    msg['Subject'] = ...



And check the raw email again. This time, lines are 76 chars long and have no padding. Data is the same, just wrapped differently as expected.

    ...
    MIME-Version: 1.0
    Content-Type: multipart/mixed;
     boundary="===============4874323426669347622=="

    --===============4874323426669347622==
    Content-Type: text/plain; charset="utf-8"
    Content-Transfer-Encoding: 7bit

    hello world, I should have an attachment

    --===============4874323426669347622==
    Content-Type: image/png
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="aaa.png"
    MIME-Version: 1.0

    iVBORw0KGgoAAAANSUhEUgAABE0AAALFCAYAAAA2rB2uAAAAAXNSR0IArs4c6QAAAARnQU1BAACx
    jwv8YQUAAAAgY0hSTQAAeiYAAICEAAD6AAAAgOgAAHUwAADqYAAAOpgAABdwnLpRPAAA/45JREFU
    eF7s/WuwdUlZ5wG+XhEUUC4ilwIRhC4uRQMFyAGhCxQsucldqhoQCl7kUtJNUVQLgkAJDU2JB2xC
    gUEBiW4aQSWEt4cQwwB6xqC+CPYHqX5jQno+jHyYCCMmZsKJmYjZs3/rnOe8uXNnrpW5LrlzrfVf
    ...



This fix *should* be extremely easy:

    diff --git "a/policy.py" "b/policy.py"
    index 6ac64a5..046b788 100644
    --- "a/policy.py"
    +++ "b/policy.py"
    @@ -209,6 +209,6 @@ default = EmailPolicy()
     # Make the default policy use the class default header_factory
     del default.header_factory
     strict = default.clone(raise_on_defect=True)
    -SMTP = default.clone(linesep='\r\n')
    +SMTP = default.clone(linesep='\r\n', max_line_length=76)
     HTTP = default.clone(linesep='\r\n', max_line_length=None)
     SMTPUTF8 = SMTP.clone(utf8=True)

The default maximum line length is indeed supposed to be 78 (characters..and I can't now remember whether I implemented it in terms of characters or octets), that's per RFC 5322.  What is wrong is that content encoded text is supposed to use a max line length of 76.

A "simple" fix would be to hardcode the maximum line length at 76 when doing content encoding.  An arguably better fix would be a separate policy control for the encoded text line length.

The former can be the bug fix, but it would be nice to have the latter as an enhancement.

Hi!

I hit this bug on python 3.4.

For the reference here are two email sources. The first one works while the second one does not. Both were produced using smtplib the only difference being the workaround proposed by Douglas Thor.
I hit this bug when using K-9 mail for reading e-mails.
Relevant github link: https://github.com/k9mail/k-9/issues/1659#issuecomment-416074768

This one works:
---------------

Return-Path: <sender@senderdomain.com>
X-Original-To: receiver@receiverdomain.com
Delivered-To: receiver@receiverdomain.com
Received: from [127.0.0.1] (smtp [127.0.0.1])
        by smtp.receiverdomain.com (Postfix) with ESMTP id 2697F23C0063
        for <receiver@receiverdomain.com>; Mon, 27 Aug 2018 09:20:15 +0300 (EEST)
Subject: Malformed base64
From: sender@senderdomain.com
To: receiver@receiverdomain.com
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Message-Id: <2697F23C0063@smtp.receiverdomain.com">20180827062015.2697F23C0063@smtp.receiverdomain.com>
Date: Mon, 27 Aug 2018 09:20:15 +0300 (EEST)

Q09OVEVOVF9MRU5HVEg6IDEyMzQ1Njc4OTBhYmNkZWYNCkNPTlRFTlRfVFlQRTogMTIzNDU2Nzg5
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OTBhYmNkZWYNCg==
----------------

This one does not:
------------------

Return-Path: <sender@senderdomain.com>
X-Original-To: receiver@receiverdomain.com
Delivered-To: receiver@receiverdomain.com
Received: from [127.0.0.1] (smtp [127.0.0.1])
        by smtp.receiverdomain.com (Postfix) with ESMTP id 0D25523C0063
        for <receiver@receiverdomain.com>; Mon, 27 Aug 2018 09:21:13 +0300 (EEST)
Subject: Malformed base64
From: sender@senderdomain.com
To: receiver@receiverdomain.com
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Message-Id: <0D25523C0063@smtp.receiverdomain.com">20180827062113.0D25523C0063@smtp.receiverdomain.com>
Date: Mon, 27 Aug 2018 09:21:13 +0300 (EEST)

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ZWYK
-----------------