Complete Nuwb struggling to set up email basics

[Desktop]

Hi I'm new to ispconfig3 and find it very confusing after using 42go. I want to set up email addresses on clients websites which use the username and password I set up previously.

In email >>> mailbox I can set up an account with pre-existing password, but can only access that mailbox with the email client username set to the email address e.g. name@mydomain.com

I need to set up mailboxes using previously set up usernames else every client would have to change their pop3 setting - can anyone help?