We assign custom posts to a regular category when publishing them. We are using Per Post delivery and recipients per email set to 1.
I had taken the site offline to a testing area, with small scale testing, I can receive email normally (CPT posts with regular category)
With large scale testing (1000+) my-name+test0001@gmail.com (these will all deliver to my-name@gmail.com) there are actually less than half of the addresses being sent email (3rd party API logging) before gateway timeouts (of course, if a page is waiting for longer than timeout it will time out)
I am using "Per Hour" option to see if this still happens - Since the whole process of actual sending and delivering 1000 email takes up to 15 minutes.