When manually setting up a mail client, for example, Microsoft Outlook, the warning message appears:
The server you are connected to is using a security certificate that cannot be verified.
The target principal name is incorrect
Do you want to continue using this server?
Pressing the "View Certificate" button shows that the mail client uses a default mail server certificate installed in Tools & Settings > SSL/TLS Certificates > Certificate for securing mail.
A separate certificate for securing mail server is selected for the domain in Domains > example.com > Mail Settings > SSL/TLS certificate for mail.
The incorrect hostname is used as Incoming/Outgoing mail server when setting up the mail client.
When securing a mail server with a separate mail certificate, only mail domains below are secured:
Set up mail client using 'example.com' or 'mail.example.com' as Incoming/Outgoing mail server.