Если я правильно понял, тебе нужно, чтобы почтовый сервер с локальным адресом мог отправлять в WAN и принимать оттуда почту. Так?
Короче, ты про сервер паблишинг правильно спросил - это все именно там и делается. Более того, там уже есть встроенный визард для организации трафика на почтовый сервер за файрволом и от него. Называется эта байда Mail Server Security Wizard.
Вот выдержка из хелпа:

Mail Server Security Wizard
Microsoft Internet Security and Acceleration (ISA) Server includes the Mail Server Security Wizard, which you can use to configure ISA Server, so that it allows hosting and securing a mail server behind ISA Server. The wizard configures ISA Server rules to securely publish internal mail services to your external users.

For instructions, see Run the Mail Server Security Wizard.

The default mail server publishing rules allow incoming traffic to the mail server. The mail server publishing rules have the following parameters:

The protocol for the selected mail service
The published Internet Protocol (IP) address of the mail server, which is exposed by the ISA Server
The internal IP address of the mail server
The Mail Server Security Wizard also creates protocol rules, to allow outgoing mail traffic. The protocol rules have the following parameters:

The protocol is Simple Mail Transfer Protocol (SMTP) (client)
The client set includes the internal IP address of the Exchange Server computer
Depending on which settings you select when running the wizard, different rules are created. ISA Server marks rules created by the wizard with a name beginning with "Mail Wizard rule". For example, if you run the wizard, and configure it to publish outgoing SMTP mail, then ISA Server creates a protocol rule named Mail Wizard rule—SMTP (client). Internal IP: IP_Address, where IP_Address is the internal IP address that you specified when running the wizard.

If the mail server is located on the same computer as ISA Server, then IP packet filters are created, rather than protocol rules.

For more information on how to use the Mail Server Security Wizard when in a deployment scenario, see Exchange Server publishing scenarios.

Запускай этот визард, указывай ему адрес сервера и рабочие протоколы - и вперед!
Если что непонатно - спрашивай еще...