Front end servers routing Internet mail

This is a very common misunderstanding about SMTP and front end servers. The truth is, SMTP does not get "front ended" as other protocols like HTTP/HTTPS (used by OWA, OMA, ActiveSync, RPC over HTTP), IMAP4, and POP3 do. In fact, SMTP running on a front end is no different than SMTP running on any other Exchange server. This means a front end server -- which by definition do not host mailboxes -- will never see any outbound SMTP traffic. You will need to make it a bridgehead for a SMTP Connector (for the address space * for outbound Internet mail) -- to route outbound SMTP traffic through it.

Inbound Internet mail is directed to the appropriate Exchange/SMTP server that the MX records in your external DNS zone(s) points to. You will need to create/point an MX record (with low preference number) to point to your front end server's external/NATted IP address to see inbound SMTP mail flow through that front end server.