A couple of years ago I wrote a series of articles on what to do when users say inbound messages aren't getting to them. I am frequently asked for help figuring out why outbound messages seemingly don't get to where they are going.
Troubleshooting outbound mail flow is relatively simple (at least to me, but then again I'm a self-professed Messaging-geek). Whenever I bring up the subject, people look at me like I'm talking about something more (dark) art than science. I think that's only true until you gain an understanding of the science. With that in mind, here's my attempt to explain some of that science and hopefully put you on the path to becoming a troubleshooting wiz.
The easiest way to think about messaging is that it is a point-to-point transmission system. It starts somewhere and wants to go somewhere else. It may require only one tranmission (hop) or several. Each transmission succeeds or fails. Sounds simple so far, right?
Let me put it another way - here's how I look at troubleshooting message flow:
First, you have to know the path a message will take to get to its destination
Second, you have to determine how far along that path it got
Third, you have to figure out why it stopped (or if it stopped)
Sounds simple enough, but as the saying goes the devil is in the details. Towards that end, here are some separate articles about narrowing the focus of your search. I'm not sure the best way to follow along. As you get used to some of these ideas you may have to go back and forth. You may want to jump to the last article in the list and see how far you can get. I'll try to link up the articles as much as possible.
Understanding your outbound mail flow
What's a Smarthost?
What's a Relay?
Understanding Message Tracking
Understanding the Queues view
Understanding the SMTP log
Understanding basic SMTP commands and responses
Using NSLookup to verify recipient system address
Using Telnet to verify SMTP connectivity
Let's Troubleshoot (putting it all together) !!