python
multipart/related mail generated with EmailMessage() class lacks mandatory MIME-Version header
Bug report
Bug description:
When creating an email with EmailMessage() that contains an HTML part with images using a multipart/related structure as the outter MIME structure, the resulting mail contains non MIME-Version header. While the only valid value for the MIME-Version header is "1.0", this is a required header field for MIME structures, see https://datatracker.ietf.org/doc/html/rfc2045#section-4
Here is a simple example code:
#!/usr/bin/python3
import email.message
svgimg='<svg xmlns="http://www.w3.org/2000/svg"><circle cx="100" cy="100" r="50" fill="green"/></svg>'
outer = email.message.EmailMessage()
outer.make_related()
inner = email.message.EmailMessage()
inner.set_content("Hello")
inner.add_alternative("<img src='cid:imgid'>", subtype="html")
outer.attach(inner)
outer.add_related(svgimg.encode(), "image", "svg", cid="imgid")
print(outer.as_string())
The output starts like this:
From: [email protected]
To: [email protected]
Subject: Hello
Content-Type: multipart/related;
boundary="===============4597207535328563093=="
[...]
It needs a "MIME-Version: 1.0" header in the main part. This will also cause some spam rating in e.g. spamassassin.
(Note: The example code uses multipart/related as the outer structure and multipart/alternative as the inner structure. While counterintuitive, this is how it is usually done by almost all mail clients. The example given in the python docs here https://docs.python.org/3/library/email.examples.html does it the other way round.)
CPython versions tested on:
3.12
Operating systems tested on:
Linux
That's interesting. EmailMessage seems to only set MIME-Version in the set_content() method, so your outer.make_related() and outer.add_related() didn't really do anything more than what MIMEPart already defines. (FYI MIMEPart is the base class of EmailMessage that does not implicitly set MIME-Version.)
Maybe we can do the same thing in the make_*() and add_*() methods too? But I am not a Python contributor, so I don't have a say... In the meantime though, adding outer['MIME-Version'] = '1.0' somewhere should do the trick. :)
