During a domain transfer, while DNS was still propogating I needed to retrieve mail from the old server to the new.
I did so thusly…
# yum install fetchmail
$ touch .fetchmailrc
$ chmod 600 .fetchmailc
I then added the following lines to .fetchmailrc
poll with proto POP3
user there with password "" is "" here
Then once ensuring you can POP3 out of the server (port 110) through the firewall, you can just run ‘fetchmail’ from the command line to grab the mail.
Topping it off with a cronjob for the duration of DNS propogation would probably be a sensible finish.