(it was spam filter blocking us - intermittently) on the email server. Some times they'd let us send 15 emails, othertimes they'd let us send 20. in a 2 minute window. Most irritating thing ever to track down. Eventually figured out if we tagged it with the "request read receipt" preference, it got rid of 90% of the issue

Strange - I have this exact set up at my old place and it worked 99% of the time. There are two things I can think off, #1 - make sure the script was using the more reliable (not bulk) email server #2 - make sure the subject line is unique in every email

We did both of those things to get rid of it. It was a few years back.