You will deal with this issue quite a bit on a daily basis. I will provide as much as I can and if you want to learn more about the commands you can send me a spark on the clock.
The first thing that you want to do is find out what email is not working and if it not sending or receiving. After that ask them how they are sending mail if they are using webmail or a 3rd party client such as Outlook or Thunderbird.
We can test the email through the server and guide them how to setup their 3rd party but not do it for them. After this the very first thing you do it CHECK the dns to make sure that it is pointed to the server. Look at the name servers, mx records, and the mail.13th-legion.com A record.
You can use leaf dns to do so. After you have verified that you want to login and check the mx entry to see if it set to local or remote. Most of the time it should be set to local unless they are using google apps. After that you want to send a test email and send it back to see if it works. If it does not send back 1 thing that is good to do is change the mail.13th-legion.com cname record in the Advanced DNS zone editor to an A record and use the ip instead of the domain.
In some cases this will solve the issue. If it does not you might have bigger problems. Also make sure to ask how long ago they set the dns because if they did it 5 minutes ago that might be your problem.
If you get here and it is still not working learn about some scripted shell commands or spark me and I will help you.