Previously, I posted about troubleshooting inbound mail flow. However, just as often (possibly more), you will be troubleshooting outbound mail flow. Hopefully, this post will help with that. As with inbound mail, there are many things which can cause problems for mail delivery going FROM one of your users to someone outside your organization. You should not take the word of a non-technical person who is reporting the problem to you as gospel. Verify the scope of the problem and ask questions such as these:
- What is the scope of the problem?
- How many people are affected? Almost as importantly, is there anyone who seems UNaffected and can still receive mail?
- Are users able to send mail between each other inside the company but not send to people outside?
- When did it start?
- Are there any error messages or common symptoms that the affected users are seeing in Outlook or other mail client?
- Are users getting any kind of bounceback message when trying to send email out? See if you can have a copy of one of these bouncebacks forwarded to you if at all possible.
- What was changed? Besides the obvious, that it was working and is now not, something may have been changed. Ask anyone whom you know may have been working on the affected mail server or domain name within the last day or so. Firewall rules? Spam filtering device or spam filtering software on the server? etc. A lot of the time, finding out what was changed will point you toward the cause of your problem.
- Check the outbound queue(s) on the mail server. If your company is having trouble sending out mail, there are probably messages piling up in an outbound queue. If you find messages in the queue(s), are they addressed to many different domains or just one or two? If just one, then there may just be a problem with the destination mail server.
- Send messages using webmail (e.g. outlook web access). Send to several different domains (e.g. your personal Gmail, Yahoo, or other addresses) to see if they go through.
- Check services/processes. Are the Microsoft Exchange services running, such as the Transport and/or SMTP services? Or if using Sendmail or Postfix, are the processes running? Sometimes, even if they are running, restarting the services/processes that deal with sending mail can correct a problem.
- Check logs in Windows/Linux for errors. For Exchange server itself, any diagnostically useful errors will be in the application log. However, keep in mind that Exchange (and mail flow in general) relies heavily on DNS functioning properly. So, you may have many errors that point to an Exchange problem, but it may just be a symptom of an underlying DNS or Active Directory issue. Check the DNS and Directory Service logs as well.
- Check the firewall. Is it blocking outbound SMTP connections from your server IP address. Use telnet to ensure that your mail server can connect outbound to other mail servers outside of your network on port 25.
- Check the remote firewall or spam filtering device. The IP address of your mail server may be blocked or blacklisted. You have a limited number of ways to determine if this is the problem. Test by initiating a telnet session to the destination server on port 25. If there is no response, try the same thing from a computer on a different Internet connection, such as your home computer. Your only other option is to get in touch with a network administrator for the destination server and see if he or she can help.
- Check DNS. Your mail server may simply be having trouble resolving DNS names to be able to deliver mail. Look up the MX records for one of the domains to which you are having trouble sending mail. Then, try to ping the DNS name for one of the MX records that was returned in the lookup. Even if it doesn’t respond to ping (your firewall may block ping traffic), does it resolve to an IP address?
- Check your reverse DNS. Going back to the outbound queues on the mail server. If there are many messages queued up, destined for various domain names, it could be a reverse DNS issue on your end. See my previous posting about reverse PTR troubleshooting.
- Check your outbound spam filter, if you have one. Some companies do, although it is rare. Beyond your mail server queue, there is another queue on the spam filter that may be filling up.
There are many moving parts when it comes to mail delivery. Answers to the pre-troubleshooting questions (top of this post) will likely help you arrive at a resolution more quickly than if you start from scratch.