Corrup mail files?

[lug at the-leveys.us (Don Levey)]

I'm noticing what seems like bizarre behaviour, and am wondering if anyone
has osme insight into it.

I'm running Sendmail 8.13.6.  One of my users tends to have a fairly large
mailbox in /var/mail/spool, as it is a very active mailing list account.
Interestingly, once it gets to about 6MB it seems to get corrupted.  I can
no longer lock the mailbox when accessing via POP3, and I get a select error
when trying to get to it through IMAP.

When deleting/renaming the file, all is well again.  The system itself seems
to rename a copy of the file with a .BOGUS extension.  I'm wondering about
two things:

1) Six MB doesn't seem all that large - why would it all of a sudden choke
at that point?
2) Is there a way of rescuing the old file?

As to #2, I haven't tried manually editing the thing and cutting it down
yet, but that's next on the list.  This isn't critical, but it *is*
annoying.
TIA,
 -Don