If the Microsoft Exchange Server Internet Mail Service has Message Delivery configured for Forward all messages to host on the
Connections tab and Diagnostics Logging is set to any level other than None for the SMTP Interface Events, you will receive the following event in the Event Viewer Application Log:
Event ID: 2003
Source: MSExchangeIMC
Category: SMTP Interface Events
Description: A new TCP/IP connection has been made to host x.x.x.x (for domain.com).
Logfile: L0000000.LOG
All messages sent out through the Internet Mail Service show the same domain name in the description.
If SMTP Protocol Log is also turned on, the event listed above will display the name of the log file where this SMTP transaction was recorded. This can be found in the \exchsrvr\imcdata\log folder. If SMTP Protocol Log is set to None the value for Logfile in this event will display None.