Howto fix emails for Cyrus LMTP and IMAP

I, as many others, have been bitten by Cyrus’ strictness when it comes to RFC-compliant email headers. Although it cost me about a full day, I still appreciate that Cyrus interpretes the RFC strictly and thus forces email to be syntactically correct. It may not strictly adhere to the “be liberal in what you accept” approach, but this way is less likely to cause problems later (with IMAP clients, indexing, searching, etc.