That said, I'm going to step onto my soapbox briefly...
I highly recommend you spend a few minutes at the outset investigating what changed in your Internet mail environment in the time leading up to the outage.
I've been through countless scenarios like this, where some investigation –- and sometimes downright interrogation -– turns up the fact that someone installed a patch, reconfigured an antispam gateway, changed ports on a firewall or otherwise made changes to a production Exchange environment without notifying the Exchange operations team.
Change control is absolutely essential if you're going to achieve any kind of predictable availability across your Exchange infrastructure. Establishing well-tested procedures, documenting processes and maintaining change logs, and avoiding rogue server re-configuration or changes, are all essential components of running a highly successful Exchange operation.
Stepping off soapbox now...
So to troubleshoot your server, try telnetting from the Internet to whatever IP address corresponds to your public Mail Exchanger (MX) record. You'll want to telnet to port 25 specifically. So if your public DNS MX record looks like the example below, you'll want to use telnet command 'telnet 192.168.1.1 25' in order to connect to the server on the proper interface and using the proper port.
mydomain.ca MX preference = 5 mail Exchanger = mx1.mydomain.ca mx1.mydomain.ca Internet address = 192.168.1.1
If Exchange is responding, you should see a banner similar to the following:
220 myExchangeserver.mydomain.ca Microsoft ESMTP MAIL Service, Version: 5.0.2195.6713 ready at Sat, 22 Jan 2005 21:02:12 -0500
Note that this banner returns the Exchange version number which should start with 5.x. If you get a banner similar to the following, namely one starting with a 6.x, then you have a problem:
220 mywindowsserver.mydomain.ca Microsoft ESMTP MAIL Service, Version: 6.0.2600.21 80 ready at Sat, 22 Jan 2005 21:05:03 -0500
The 6.x indicates that this is actually not an Exchange erver responding, but rather a Windows machine running the IIS SMTP Service (in this case, it's a Windows XP machine).
So if you haven't guessed by now, the first step in troubleshooting your problem is to confirm that your Exchange server is, in fact, listening on the expected IP address and is answering connections to port 25. I've seen numerous cases in which an alternate service, such as IIS SMTP Service or an antispam gateway, is intercepting port 25 calls unexpectedly.
Once you've ruled that out, you'll want to try your telnet test, but this time use the following commands:
Subject:Test Message at 1:01 p.m.
This is a test.
Feel free to replace the "me.com" with any Internet domain. Make sure the "RCPT TO" address is a valid address on the server.
There are a few things this test will show you. First, if the server is returning any errors during SMTP transmission, you'll see them in the telnet session output. Second, once you've completed the message delivery, you should see the message show up in the target mailbox. If you do, everything is fine from an Exchange perspective and you need to check whether a firewall or other intermediary device is blocking SMTP connectivity to your server.
If you don't see the message in the target mailbox, you'll want to launch Exchange Administrator (Exchange System Manager for Exchange 2000 or 2003 environments) and use the message tracking center to track the message from email@example.com. It's likely sitting in a queue somewhere, in which case you've found your problem and will need to investigate exactly why mail is queuing instead of being delivered.
There are a lot of steps involved, because so many variables could be causing your mail delivery issues. But I've shown you the key steps to take to troubleshoot your inbound Internet mail issues.
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