Hello Cloudron team/patrons,
I was going to ask for help, but we came up with the solution, so we'll pass it forward.
We needed to recover from a system outage, after temporarily running out of disk space. Besides corrupting the certificates (already fixed thanks to the forum here --thank you), we also borked the outgoing SMTP mailserver.
The obvious symptom from RoundCube showed "SMTP Error (-1): Connection to server failed."
Reviewing the 'mail' service logs revealed that there was a corrupted "queue file" (zero bytes), which was crashing the service.
To fix this issue, SSH into the machine, navigate to
/home/yellowtent/boxdata/mail, move the
haraka-queue to a backup location, and recreate that directory. After that, restart the mail service.