If you're migrating a domain, sometimes their mail will go to the old server for a while after you've changed the DNS. You can move their mail to the new server by following these steps:
1) Go to the user's Maildir directory
cd /var/qmail/mailnames/<domain>/<user>/Maildir
2) Tar their mail directories
tar cvzf <user>.tar.gz cur new tmp
3) Move to a web accessible location
mv <user>.tar.gz /home/httpd/vhosts/<web-accessible-domain>/httpdocs/
4) Log onto the second server and go to the user's Maildir directory
cd /var/qmail/mailnames/<domain>/<user>/Maildir
5) Retrieve the user's mail tar file that you created
wget http://<web-accessible-domain>/<user>.tar.gz
6) Un-tar the files to their correct locations
tar xvzf <user>.tar.gz
7) Remove the tar file
rm <user>.tar.gz
Go to the original server and remove the tar file
rm /home/httpd/vhosts/<web-accessible-domain>/httpdocs/<user>.tar.gz
Thank you