What’s the difference between using mail.domain.com and imap/smtp.domain.com?

When selecting what credentials to use within your mail client, you have a choice between using either: mail.yourdomain.com or imap/smtp.yourdomain.com

There is no difference between using either of those options.

If you use mail.yourdomain.com it is likely you’ll get a “mail certificate name mismatch” / “cannot verify server identity” error. This is explained in the guide here.