I have a Cisco ASA 5510 that is not in production yet, and I am attempting to set up NAT for smtp, using ASDM. I have done this on our current firewall, which is not a Cisco, but the process was more intuitive.
I have established a static NAT rule for outgoing traffic as follows:
On the internal interface the source address is 192.x.x.5 (the mail filter) and the destination is "any."
On the external interface the translated address is 76.x.x.30, which our public IP for our MX record, the external interface's address being 76.x.x.31.
PAT is enabled for smtp.
ASDM 6.0 will diagram the rule, and it appears correct.
I am not entirely certain about how to set this up in reverse, however, since "any" cannot be used as a valid source. Should the MX IP be the source?