In the light of the comments exchanged above, I'll write up my submissions as a full answer.
You say you're running dovecot as a POPd/IMAPd at the same time, and I've suggested that it's possible that the second client, corr, could be running a POP client which periodically swoops in, retrieves, and deletes a group of emails.
After some tests, you also think this is a client-side issue, and ask if dovecot can be told not to allow that client to delete emails (you ask about postfix, but this isn't postfix's problem; postfix's job was finished when that email hit the INBOX).
The short answer is that I don't think there is, and if I'm wrong I very strongly suspect that any change would apply to all users, not just one. But there are some possible workarounds.
corr the only user using POP? If (s)he is, consult on a migration from POP to IMAP. IMAP is a vastly more sophisticated method of reading email, which (by default) leaves the read email on the server instead of downloading it to the client.
Are you doing this for audit purposes? If so, would it be easier to have everything that's delivered to
corr also delivered to a second user on the same system?
If this is a simple issue with
corr wanting to be able to read email on the server whilst (s)he has a client downloading and deleting it, then I'm afraid the real answer is to tell the user to reconfigure their client not to do that; she can't really complain if the server is doing exactly what (s)he asked it to do.