Everything worked fine for me. Unexpectedly it stoped sending messages. Client program (e.g., FairMail on Android) reported an ERROR saying “it is SSL failure”, “failurel in the protocol”. I’ve tried to use TLS/465, STARTTLS/587 with no success. then tried to use IP except domain name (e.g., 23.99.123.93, but not mail.example.com), and it has started working, mails are sent.
At the moment I cannot understand the root of the problem, how to back to using domain name, not IP address of the server in my clients’ configuration?
I would very very appreciate any ideas, helps, talks on this matter.
By the way, I use mailcow behind Nginx reverse proxy. Certificates are installed with certbot using --nginx module. I didn’t change anything up until error has been arisen.