Yesterday I was setting up a new server for a client / friend. During the setup, we had a brief period wherein mail was delivered to mbox instead of to Maildir. That is, we had mail sitting in /var/mail/USER instead of ~USER/Maildir, for a number of values of USER.
At first, I tried using formail and procmail to send it to the local user. I did this using a method I found using Google. In short it involves adding this to ~/.procmailrc:
:0 * ! firstname.lastname@example.org
This is the address to which you wish to forward the mail.
Then you issue this command:
sudo cat /var/mail/USER | formail -s procmail
Where USER is the email mbox that you wish to forward to email@example.com
NOTE: I tried forwarding the mail to the local user (e.g. I put just foo in ~/.procmailrc but the mail server complained about a mail forwarding loop. I’m sure there’s an email way around that problem too. But in this situation, I forwarded the mail to other servers, such as gmail or yahoo.