There is NO problem.
You received the test email? Yes. Than it means that email works from our side.
The content of test email is boilerplate template test email with text addressed to forum admins from Discourse and you just ignore it.
Did you click on links in that email? Then you found out that your ISP IP is blacklisted. But it has nothing to do with ability to receive email.
The point was if you are getting the TEST EMAIL.
BTW: the IP of server did NOT change and no, our server is not on any black lists.
Update : I just tested with testing gmail account and it worked just fine.
It was slow (i.e. it took Google over 2 minutes to "notice" that email was sent but it arrived). Any other account than Gmail works in one second.
Update 2: I found some emails to be still in the queue deferred for later delivery. This could happen if more than 512 emails per hour are attempted. Such emails are not lost they are just deferred for later delivery to avoid "flood" of email that may cause recipient servers to think that we are trying to "attack" them. Since many people are using GMail and many users are subscribed for "all" topic updates this rate limiting mechanism can kick in.