Issue Mail issue: Remote server returned '554 5.7.4 < # 5.7.4 SMTP; 554 5.7.4 failed to deliver message via SSL/TLS. >' -lost connection after STARTTLS from

[mcac]

Server operating system version

Almalinux 8

Plesk version and microupdate number

18.0.65 #2

Hello,
a customer has a problem receiving e-mail from only one sender.
The sender receives a bounce email with the error:
Remote server returned '554 5.7.4 < # 5.7.4 SMTP; 554 5.7.4 failed to deliver message via SSL/TLS. >'

I can see the error in the mail log:
lost connection after STARTTLS from subdomain.domain.com[XXX.XX.XXX.XX]

It seems that the sender's mail server has a problem with the SSL certificate on our mail server, right? But the certificate is correctly installed and valid. It also only affects the one sender.
Important to know, the domain has an A-record and AAAA-record to another web server. We have therefore set up a subdomain for mail communication: mails.domain.com

What could be the problem?

 

[scsa20]

Hard to say without knowing the full context and how everything is set up, all I can say is make sure that the certificate that's installed supports the subdomain's DNS record (or you are using a wildcard) and is securing mail access.