If I understand correctly, I guess you can... however, SMTP Inbound connections of SMTP will always use the first NAT. Look at the example below:
object network smtp-server
host x.x.x.x
object network NAT-IP-1
host y.y.y.y
object network NAT-IP-2
host z.z.z.z
object service SMTP
service tcp destination eq 25
Nat (inside,outside) source static smtp-server NAT-IP-1 service SMTP SMTP
Nat (inside,outside) source dynamic smtp-server NAT-IP-2
That way, all inbound traffic for SMTP will use NAT-IP-1 and the rest of the outbound traffic from STMP server will use NAT-IP-2.
Let me know if that is what you needed.
Mike
Mike