Username or Account ID: firstname.lastname@example.org
Password: As you set it when adding the account to plesk.
Incoming/POP/IMAP server: mail.'domainname.co.uk*'
Outgoing/SMTP server: mail.'domainname.co.uk*'
* Where domainname.co.uk is replaced by your domain name for the email account.
My Outgoing server requires authentication (On), to use the same username and password as incoming mail server.
Use SSL (Off), this is not required.
SMTP: 25 or 587
Note: Your server with Nimbus is capable of being used as a basic mail server, but the ongoing battle between spam management, hacking and blacklisting means that Office 365 and Google Apps are the go-to platforms if email is critical to your business/site. Read more here.