at router You may write next:
ip nat inside source static tcp 110 110
ip nat inside source static tcp 25 25
and, add permit rule in "in" ACL if it exsist.
What software version on ASA?
For 8.4 it is may be like this (192.168.0.10 - SMTP server and 192.168.0.20 POP3 server):
object network SNMP_SRV
host 192.168.0.10
nat (inside,outside) static interface service tcp smtp smtp
object network POP_SRV
host 192.168.0.20
nat (inside,outside) static interface service tcp pop3 pop3
object-group service POST_SMTP tcp
port-object eq smtp
object-group service POST_POP tcp
port-object eq pop3
access-list 101 extended permit tcp any object SNMP_SRV object-group POST_SMTP
access-list 101 extended permit tcp any object POP_SRV object-group POST_POP
where ACL 101 it is "in" ACL on Outside interface
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Helping seriously ill children, all together. All information about this, is posted on my blog