cisco - Trying to setup NAT from 2 outside IPs to the same private IP

Cisco ASA 5510
I currently have a NAT for SMTP on one outside IP to an internal IP. I need to setup 2 external IPs to NAT to the same IP internally. How can I do that?
ex:
10.10.10.1 25 --> 192.168.0.200 25
10.10.10.3 25 --> 192.168.0.200 25

Answer

You won't be able to use static PAT for this as you would break the 1:1 mapping rule. Firewall has to know what mapping to use in both directions - both in->out and out->in. In your case if 192.168.0.200 originated connection from port 25 firewall would not know which global IP to use. In other words, it's not possible this way.

Easiest solution would be to assign additional IP address on the internal device and keep the NATs clean. Let's say you assign additional IP of 192.168.0.201. Configuration would be:

static (inside,outside) tcp 10.0.0.1 25 192.168.0.200 25
static (inside,outside) tcp 10.0.0.3 25 192.168.0.201 25