It seems some tools try to interpret the filename of message files,
even though they shouldn't:
"Do not try to extract information from unique names."
In particular, they seem to interpret the first part of the name (before
the first dot) as the # of seconds since the Unix epoch (ie.,
time(NULL)). That's not what mu/mu4e put there.
So, let's conform a bit more to the expected filename (as per the
maildir spec), so we're not confusing those tools.
a350e2047d