If mb2md stumbles upon a "From " line with "@" replaced by " at ",
e.g. like this one:

…, then it throws the following perl warning twice:

It still seems to have recognized the mail borders properly, but sets
the file date incorrectly:

This can be the case if the mbox contains mails which are imported from
Mailman archives (or the downloaded more-or-less mbox generated by
Mailman). The above example is contained in
https://lists.alioth.debian.org/pipermail/aptitude-devel/2012-January.txt.gz