If you host your own mail server, you can easily use HeatShield to configure your firewall for email.
Specifically, you will want to open the following ports:
First, connect your server to HeatShield and upgrade it to use custom rulesets.
Then, open your Rulesets page and click Create Ruleset.Next, name your ruleset and click Create Ruleset.
Now, create a new firewall rule by selecting the policy, the destination, and the source.
Set the Policy to Allow and the Source to Any.
Select Custom from the Destination dropdown and enter TCP 25 as the first SMTP port. Click Add.
Repeat this step for the rest of the email ports you need to open on your server.
Your final ruleset will look similar to this example:
To apply your new ruleset to your mail server, open the server in HeatShield.
Select your Email ruleset from the drop-down list and click Apply Ruleset.
Your new ruleset will be applied.