This list is probably not complete.
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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).
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Backslashes in atoms (RFC 5322, 3.2.3) are parsed as if they were inside quoted strings.
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Return-path is accepted without angle-addr (RFC5322, 3.6.7).
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Encoded words within quoted strings (RFC2047, 5.3) are decoded for header printing.
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Encoded words within MIME parameters (RFC2047, 5.3) are NOT decoded.
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Empty encoded words are decoded as empty string (RFC2047, 2).
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Split multi-octet characters between encoded words (RFC2047, 5.3) are reassembled if the encodings agree.
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Date parsing is strict, obsolete timezone and two-digit years are not parsed (RFC5322, 4.3).
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Mails without MIME-Version (RFC2045, 4) are still subject to MIME decoding if the Content-Transfer-Encoding header is present.