First off, decode text portions of messages since some archived
mail I got was converted from quoted-printable or base-64 to
8bit by the original recipient. Attempting to merge them with
my own archives (which had no conversion done) led to
unnecessary duplicates showing up.
Then, normalize CRLF line endings in text portions to LF.
In the headers, we relax the content_id hashing to ignore quotes
and lower-case domain names in To, Cc, and From headers since
some mail processors will alter them.
Finally, I've discovered Email::MIME->new($mime->as_string)
does not always round-trip reliably, so we calculate the
content_id twice on user-supplied messages.