Plesk general email settings for your mail program.

Username or Account ID: alias@domainname.co.uk
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.

Security settings: 
My Outgoing server requires authentication (On), to use the same username and password as incoming mail server.
Use SSL (Off), this is not required.

Default Ports: 
POP: 110
IMAP: 143
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.