SSL security certificates are complex and often poorly implemented. In your example the email server host name in the server security certificate differs from the server's actual name. That's a big red flag and should be taken seriously.
The right way to address the problem is to have "Roundcube" address that discrepancy. They should either deploy their security certificates properly, or not at all.
If they are not willing to do that, then you must decide whether the certificate is trustworthy or not. Assuming you want to trust it despite its improper implementation, then just click Continue and accept the risk that Mail is connecting to the correct email server despite the host name mismatch. Mail won't ask you again.
An improperly implemented security certificate is arguably worse than no security whatsoever. Lacking a properly implemented security certificate, they're offering the illusion of security.
I wonder if anybody can give details of incoming and outgoing server settings and port selection etc?
That's a question for your email service provider "Roundcube", but you might find the following useful: Mail Settings Lookup - Apple Support. No one else will be able to know those details, unless they're familiar with "Roundcube" and I"m not.
