Subject: | No MIME-Version header field set, but RFC 1521 requires it |
When building MIME mails, e.g. Content-type: multipart/alternative for Text+HTML mails, no "MIME-Version: 1.0" header is added automatically, which is required by RFC1521.
Some mail/groupware servers are not able to process such mails correctly, e.g. if they their policy forbids HTML content. The user then gets the pure MIME encoded content, with all delimiters, applied quoted printable encoding etc., rendering the mail unreadable.