The details required to access a mailbox from software on your computer are as follows.

The username to use to access an email account is always the full email address that you are trying to access.

The mail server setting for incoming and outgoing mail is always mail.yourdomainname.com

The password will be the password you selected when you created the mail box.

If you're not sure please check the details in your control panel in the "Email Configuration" section.