[Imap-uw] mail moved from /var/spool/mail/username to ~username/mbox , only for some users?

[Yiorgos Adamopoulos]

Your binaries were compiled with

EXTRADRIVERS=mbox

This means that if ~/mbox exists, then whenever a program linked with
c-client (like imapd, ipop3d, alpine) accesses the mail spool
(/var/mail/username) mail is copied from there to ~/mbox. The
combination of those two files is INBOX. If ~/mbox does not exist,
then mail is processed as expected.