Question

I work on a server that processes email, and as part of that, we do some MIME parsing/encoding. I’ve recently had an issue arise for a message that is valid otherwise, but contains a Latin-1 character in a MIME header. Someone entered an e-mail address to multiple recipients containing a Latin-1 character, so the SMTP envelope only contains the valid recipients, but the To line still contains the invalid address and improperly-encoded string.

It was my impression that this is illegal, and that MIME headers are required to be 7-bit. 8-bit values in MIME headers have to be encoded in the form

=?charset?encoding?encoded text?=

The header in question is something like this:

To: <changé[e-mail]>, <[e-mail]>

My question is: Is this valid MIME and I just don’t know about it?

Answer 1

From RFC2822, Internet Message Format, section 2.2, Header Fields:

Header fields are lines composed of a field name, followed by a colon
(":"), followed by a field body, and terminated by CRLF. A field
name MUST be composed of printable US-ASCII characters (i.e.,
characters that have values between 33 and 126, inclusive), except
colon. A field body may be composed of any US-ASCII characters,
except for CR and LF. However, a field body may contain CRLF when
used in header "folding" and "unfolding" as described in section
2.2.3. All field bodies MUST conform to the syntax described in
sections 3 and 4 of this standard.

Therefore, any non-ASCII characters are illegal.