Mime-Version header case is incorrect and receives spam score weighting

[gav-]

mimetic is producing the MIME-Version header as "Mime-Version" (note the difference in letter case for MIME). This is technically incorrect (see: https://www.w3.org/Protocols/rfc1341/3_MIME-Version.html), and causes anti-spam engines to add a weighting to the spam score for emails with this incorrect header in it. (Specifically I am using Rspamd, but it is likely to apply to others too).

I hope this report helps improve the quality of the mimetic library!

[jktjkt]

The format of e-mail messages is defined in RFC 5322 which says that (emphasis mine):

This specification uses the Augmented Backus-Naur Form (ABNF) [RFC5234] notation for the formal definitions of the syntax of messages. Characters will be specified either by a decimal value (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by a case-insensitive literal value enclosed in quotation marks (e.g., "A" for either uppercase or lowercase A).

A TL;DR version of that is that all e-mail header names are case insensitive. A software could generate something silly asmimE-verSIon, and it will still be standard-compliant.

Also, and just FYI, the actual content of the MIME-Version header is subject to the usual 822-style comment removal. There's even an example in chapter 4 of RFC 2045, on page 10 they show that all these are identical:

• MIME-Version: 1.0
• MIME-Version: 1.0 (produced by MetaSend Vx.x)
• MIME-Version: (produced by MetaSend Vx.x) 1.0
• MIME-Version: 1.(produced by MetaSend Vx.x)0

[Basskrapfen]

There is also an discussion on stackoverflow which comes to the same conclusion. When following the RFC the mail header names should be case insensitive.
https://stackoverflow.com/questions/6143549/are-email-headers-case-sensitive

But it seems there is software out there which is not 100% standard conform and checks for the casing of the "MIME-Version" header. In the stackoverflow discussion Thunderbird is also mentioned and the OP does have problems with spam detection software.

So maybe it would be nice to change mimetic to also generate "MIME-Version" even when the spec says it should not matter.

[jktjkt]

The SO post is 10 years old, hopefully TB got fixed in the meanwhile. Also, it is not really just Mimetic, apparently Discourse is generating Mime-Version, too. Anyway, someone already asked rspamd about this: rspamd/rspamd#3923, there's no specific conclusion in there, just a statement from maintainer with no backing data.

Personally, I don't really care either way. There's a ton of broken SW when it comes to e-mail producing, e-mail handling and e-mail parsing. In the MIME realm, I've seen producers split UTF-8 multibyte strings into separate encoded-words, for example. I just wanted to reply to an unanswered issue where the OP provided an inaccurate description.