Standard-violations detected in the wild during development of mblaze

This list is probably not complete.

RFC5322 assumes CRLF line endings throughout, but Maildir messages are generally using Unix line endings. mblaze accepts both, and only uses CRLF when required (e.g. for signing).
Backslashes in atoms (RFC 5322, 3.2.3) are parsed as if they were inside quoted strings.
Return-path is accepted without angle-addr (RFC5322, 3.6.7).
Encoded words within quoted strings (RFC2047, 5.3) are decoded for header printing.
Encoded words within MIME parameters (RFC2047, 5.3) are NOT decoded.
Empty encoded words are decoded as empty string (RFC2047, 2).
Split multi-octet characters between encoded words (RFC2047, 5.3) are reassembled if the encodings agree.
Date parsing is strict, obsolete timezone and two-digit years are not parsed (RFC5322, 4.3).
Mails without MIME-Version (RFC2045, 4) are still subject to MIME decoding if the Content-Transfer-Encoding header is present.