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.
Primarily, we allow you to use mail.yourdomain.com so that you don’t have to give your customers any instances of ‘stack’, creating a more customised experience.
If you use mail.yourdomain it is likely you'll get a “mail certificate name mismatch” / “cannot verify server identity” error. This is explained in the guide here.