[FIX] mail: more robust parsing of In-Reply-To/References (OPW 608919)

When parsing incoming messages, ignore white-space around
In-Reply-To headers, and extract message-id items inside
the References header using a regex.
This actually serves as a workaround for broken MTAs
mangling References (such as outlook.com nesting past ones
with commas, violating RFC2822).

Closes #516 as a manual rebase.