Before I upgraded to Debian Stretch, I had to manually install ISP-Config and adjust a bunch of other things before I could even install it. Now, when I went to check for a new version to run on Stretch, I was relieved to learn that they had built an automated script which is on GitHub.
So, I set up my E-mail with nearly the same configuration as my old one. I thought everything was fine after that. I installed WordPress, and I've been able to get WordPress notifications, like when I tested my contact form plug-in, and a messaged popped up in my inbox.
I also verified that I could send E-mails using Roundcube and Thunderbird. I got the message in my G-mail inbox. But, when I replied to the message, I didn't see it in my server.mydomain.com inbox, and I got no error from Google about it not being delivered. Also, I didn't see any drafts, junk, or trash folders in my account following the setup of my mailbox. I have found some log entries that may or may not be helpful. I found it in
/var/log/mail.log, which was updated recently.
Jun 6 15:08:07 server postfix/smtpd: warning: hostname remote.mpsukltd.com does not resolve to address 188.8.131.52 Jun 6 15:08:07 server postfix/smtpd: connect from unknown[184.108.40.206] Jun 6 15:08:10 server postfix/smtpd: warning: unknown[220.127.116.11]: SASL LOGIN authentication failed: UGFzc3dvcmQ6 Jun 6 15:08:10 server postfix/smtpd: lost connection after AUTH from unknown[18.104.22.168] Jun 6 15:08:10 server postfix/smtpd: disconnect from unknown[22.214.171.124] helo=1 auth=0/1 commands=1/2