I am running an Exchange 2003 Enterpise server and am having some issues with sending calendar events to external users. The email seems like it will go, but then gets dropped during the transmission. I have tested sending to the same domain with another Exchange server and it works properly, so I feel that the sending server is the issue. Any ideas/suggestions?

Have you got message tracking enabled? If so, track the message and see what happens. If it gets stuck somewhere post back up here.

I get the following when SMTP logging is on: Event Type: Information Event Source: MSExchangeTransport Event Category: SMTP Protocol Event ID: 7000 Date: 10/14/2007 Time: 2:51:00 PM User: N/A Computer: SERVERNAME Description: This is an SMTP protocol log for virtual server ID 1, connection #26832. The remote host "mail.server.com", responded to the SMTP command "rcpt" with "250 2.1.5 <tyler@domain.com>... Recipient ok ". The full command sent was "RCPT TO:<tyler@domain.com> ". This is an informational event and does not indicate an error. The message will make it to my server (the remote server), but it does completely send all the data. I never get a quit command from the sending server on my server (the remote server). A new network security appliance was just installed and I didn't now think of this before. I am going to eliminate this as a possibility and re-post.

Your next step is to track the message though, beyond SMTP logging with message tracking. http://www.msexchange.org/tutorials/The_Exchange_Message_Tracking_Center_or_How_to_Save_Your_A_in_a_Pinch.html