What mail server software are you running?
User account names/passwords can be moved over by doing the following.
tar password.tar /etc/passwd
tar shadow.tar /etc/shadow
replace the one's on the new machine with the copies from the old machine.
User accounts.
tar home.tar /home
gzip home.tar
move it over and then gunzip/untar and move accounts over to the new /home partition.
HTH
User account names/passwords can be moved over by doing the following.
tar password.tar /etc/passwd
tar shadow.tar /etc/shadow
replace the one's on the new machine with the copies from the old machine.
User accounts.
tar home.tar /home
gzip home.tar
move it over and then gunzip/untar and move accounts over to the new /home partition.
HTH