Email for site owner users may be found either under
/var/spool/mail or
/home/USERNAME/Maildir, depending on whether you're using mbox or Maildir.
Email for virtual email boxes (per domain) may be found either under
/var/spool/mail/virtual, or
/home/USERNAME/imap, again depending on whether you're using mbox or Maildir.
I'm not sure what kind of domain information you're looking for; there's lots of domain information spread through the system. You can find a lot of it with this command, from your shell:
Same with username; you can find information spread through the system and also in
/etc/passwd, and in CentOS and RedHat Enterprise Linux, in
/etc/shadow. And lots of other places as well.
Passwords in both Linux and Unix are one-way encoded; there's no way to find them; you'll have to change them if you forget or lose them.
If you're more specific as to the kind of information you're looking for, it may be possible to help you further.
Jeff