I have been having this issue for at least a week and possibly much longer, but as far as I know, nothing has changed on my end and this been working for more than a year.

Essentially, Dynu is failing to deliver email to my email server. It's sitting in the mail queue of the Store & Forward. This is whether I send the email from Dynu, Google, or my own system.

I have several domains that come through Dynu, all going to the same mail server (my own, hosted on my NAS). For some reason, when I send an identical test email to all my domains, it may come through on one of the domains, but not really less than an hour later. Others, despite going to the same mail server, are stuck in the Dynu queue. Additionally, sometimes one of the emails in the queue will be delivered, but not the rest of them, with no explanation.

I have already tried using ports 25, 465, and 587 with various SSL/STARTTLS settings, including doing a different port for each account. It seems like changing the port, sometimes, will cause the mail to start flowing, but it's the change and not the specific port.