Yes, I noticed the delay settings. My upstream provider has a standard 200 email per hour limit on their SMTP server (this is very low!) which can be upped to 600 / hr with a bit more $.
I did a quick script to test this limit through php's mail() function and sent over 600 emails in 5 minutes without issue. Now wheter or not I will hear about this or not is another question. I did not notice any server load increase in WHM while running my little script.
So this suggests that the mail() function bypasses the SMTP limit that they have set. Does this make sense?
Does UE use mail() in the same manner?
Does the UE log track when the emailings fail, at what point and why?