@ianhyzy I registered a new domain just for this purpose. It's not ideal but I don't have to use a relay so it works out in the end. I'm hosting with DigitalOcean so the PTR record is set by changing the server (droplet) name to match the mail server domain. Last I checked my headers, all were good and pointing to the new domain setup.
Edit: BTW, I've used Amazon SES for my newsletters in the past and they're pretty cheap. IIRC, I sent like 9k emails for under $1 USD.
@girish & @nebulon - It looks like it didn't fix it (the steps from the other page), as I still see it in the logs at 2020-10-27T18:06:51.000Z which is UTC and translates to 11:06:51 AM Pacific Time, well after I followed the earlier steps which was as of 10:54 AM Pacific.
@girish thank you for the insight. If I am extending the existing drive then I'll make sure to keep it SSD as it already is, I may try HDD if going the first route though. I'll check, this isn't an easy task for extending the existing disk unfortunately, but this may be the better move. Of course the drawback there I guess is I'll be paying for way too much disk space and need to do it all over again if I for example lose one of the clients that are using so much email space (as email is the single biggest consumer on my server right now, by a lot). I'll have to run some tests.
@nebulon That's not to say that I don't think Cloudron shouldn't receive webhooks for other reasons. I'm surprised you aren't using them for your multi-cloudron dashboard actually (to update the status of "subcontainers" from all servers instantly).
Some technical details on the issue. In 5.6, we allow the mail server name to be set and for this we introduced a database field called mail server name. For some reason, if an update only went through half way, the database migration scripts did not complete fully and this mail server name field is not populated in the database. This in turn results in the mail container not getting the correct certificate.
The first command /home/yellowtent/box/setup/start.sh is running the database migrations. The second command is restarting cloudron code. The third command is re-configuring mail container with the correct certificate.