Yes, this is very much possible on PIX.
Lets assume that your internal mail server is 10.10.10.10 and the public IP for this mail server is 2.2.2.2. Now, you want to use 2525 for smtp and 1100 for POP3. For this, you can use following commands-
static (inside,outside) tcp 2.2.2.2 2525 10.10.10.10 2525
static (inside,outside) tcp 2.2.2.2 1100 10.10.10.10 1100
access-list outside_in permit tcp any host 2.2.2.2 eq 2525
access-list outside_in permit tcp any host 2.2.2.2 eq 1100
access-group outside_in in interface outside
clear xlate
I have assumed that "outside_in" is the access-group applied on the outside interface of PIX.
If you dont have a separate public IP address for the mail server and want to share the outside interface IP of PIX, commands would change slightly:
static (inside,outside) tcp interface 2525 10.10.10.10 2525
static (inside,outside) tcp interface 1100 10.10.10.10 1100
access-list outside_in permit tcp any interface outside eq 2525
access-list outside_in permit tcp any interface outside eq 1100
access-group outside_in in interface outside
clear xlate
Hope that helps.
Regards,
Vibhor.