Anyway to track intermittent email issues on a VPS?

I have a hostgator VPS with custom nameservers and I have had two clients now that have had issues where they will send an email that is just never received. No error or anything, I have called Hostgator numerous times and they send a test email from the account and everyhting is fine. What could be going on?

You need to find out the exact chain of events, the time of activity, the recipient and the sender, then check the mail server logs at these times to see what occurred. You may need to turn up the logging granularity to check future disappearing emails. The support you are getting is typical of what to expect from cheap hosts unfortunately (minimum effort to pass the buck)

Thanks for your advice! We did start checking through logs but then I lost my phone connection …lol. I have been toying with the idea of moving to media temple but they charge a lot of money to move hosts and I dont know what would happen with my WHMCS set up…