Corrupt mail files

[david-8uUts6sDVDvs2Lz0fTdYFQ at public.gmane.org (david-8uUts6sDVDvs2Lz0fTdYFQ at public.gmane.org)]

As you know, I recently rebuilt my server with Ubuntu Jaunty, including
Postfix and Dovecot for SMTP and IMAP.  Twice so far I've run into trouble
where my wife's mail (and ONLY my wife's mail) in /var/mail/susan is
getting corrupted.  The beginning of the file is a series of zeroes, so
SquirrelMail is bitching about it not being a valid mail file.

Note that my /var/log/david is about 400X the size of hers, and mine is fine.

~# od -ch /var/mail/susan | head -10
0000000  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
        0000 0000 0000 0000 0000 0000 0000 0000
*
4560000   (   S   t   r   o   n   g   M   a   i   l       E   n   t   e
        5328 7274 6e6f 4d67 6961 206c 6e45 6574
4560020   r   p   r   i   s   e       3   .   2   .   3   .   3   (   3
        7072 6972 6573 3320 322e 332e 332e 3328
4560040   .   0   0   .   3   2   0   )   )   ;       T   h   u   ,
        302e 2e30 3233 2930 3b29 5420 7568 202c
4560060   1   1       J   u   n       2   0   0   9       0   9   :   0

This never happened before, and I'm trying to figure out what the culprit
could be.
- It could be procmail, but that seems unlikely to me. .procmail doesn't
even have an explicit thing that writes to /var/mail/susan.
- It could be Dovecot or Postfix, but they worked fine for me under Fedora 8.
- It could be that I'm using ext4, which might not have been a smart thing.
- It could be that I'm using MBox instead of Maildir, something I've been
meaning to change.  But that doesn't affect the inbox, just the saved IMAP
folders, right?
- It could be SquirrelMail, but why would it corrupt her mail and not mine?

This is seriously affecting the WAF for the home server, so I need to fix
this post-haste.  Any advice would be appreciated.