Incoming SMTP blocked by ISA

I ran into an issue with my recently deployed ISA firewall. It was blocking
any STMP incoming traffic. This made it so my Exchange users could send 
email just fine but could not receive any incoming email. I checked the 
Event Viewer and saw this:

Event Type:        Warning
Event Source:    Microsoft Firewall
Event Category:                None
Event ID:              14090
User:                ...

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