[exim] Deliver mail to another file if quota is full

Hi,

here we have a policy not to bounce any mail.

Appendfile delivery takes place to /home/${local_part}/.imap_mail/inbox
If the user is over filesystem quota, the mail should be
stored in /var/mail/${local_part} instead.

How to achieve this with Exim 4.63, packaged by Debian Etch?

Our imap/pop3 daemon will then try to snarf the mails
from /var/mail to $HOME after the user deleted some files
to comply with the quota settings.

Greetings, Bernd Kuhls