The Server Diagnostics feature of the WebAdmin allows you to execute @Mail's test module, testatmail.php, without needing to go into the server console shell. The output of the feature can give you insight as to if all services needed by @Mail are running - say, POP3 and SMTP - and display any issues the system might be experiencing.
To activate, just choose a domain from the server-generated domain list, then click on "Start". The module will then start, and generate its output as a list on the WebAdmin window.