email missing: NFS or sendmail

I have very strange problem:
One user on my server is complaining that he occasionally loses e-mails. On one particular occasion, he read an important e-mail, and after a few minutes it disappeared. He uses Netscape Mail&Newsgroup with IMAP settings.

The servers are a little complicated. The email server is using FC3 sendmail/dovecot with latest updates. The user's inbox is located in the folder
/var/spool/mail/, which is physically located in the server. The users' mail folder, where .imap/ folder locates, is in another machine and mounted to the server with NFS. That machine is using RH7.3, Kernel2.4.18-3

Can anyone give me some help on this?
Thanks,
Bob

can you check the logs and see if dovecot is reporting issues with locating the mail?

Thanks for the reply.
What logs should I check?