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

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...

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