[Wolves] Couple more mail questions

[Wayne Morris]

If you run two mail servers, one main + one backup and mail is received 
at the backup then would I be right in thinking that the
backup receives ALL mail for the domain ie AAAA at domain.com thru 
ZZZZ at domain.com including mail to valid usernames.
Then when the time comes for A to receive the mail from the backup, it 
rejects all but valid users leaving the backup to send reject emails 
back to the originator.
However, if the originators mails were spoofed, you end up with a load 
of frozen bounces on you system?
This is the way I understand it, anyway around it?

Second Q, is there an easy way of migrating mailboxes from one system to 
the other or better still having a live copy of both on each box -
only a handfull of users.

Cheers