I run a Mailcow-Instance under eg. mail.main.com and have configured another Domain secondary.com under “Configuration –> Mail Setup” which I’d like to use for sending/receiving mails.
Adding a new account to Thunderbird which checks the certificate of secondary.com:143 results in an error:
The certificate belongs to another website. (Screenshot). Ignoring the certificate error and sending a mail to some Mail-Tester Website results in a decent score.
openssl s_client -starttls smtp -crlf -connect secondary.com:143 gives following output:
Didn't find STARTTLS in server response, trying anyway...
140555206051136:error:1408F10B:SSL routines:ssl3_get_record:wrong version number:../ssl/record/ssl3_record.c:331:
no peer certificate available
No client certificate CA names sent
SSL handshake has read 228 bytes and written 342 bytes
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
I am running the WebUI behind the traefik reverse proxy und use certdumper to extract and insert the certificate (of mail.main.com!) at
Further i appended a screenshot about my DNS-Configuration which seems to be fine.
Any help appreciated 🙂, since I am running out of ideas