The myWeb server, through the DirectAdmin control panel allows you to create e-mail addresses in your sub-domains or additional domains. Addresses can be full e-mail mailboxes or forwarded to other mailboxes.
You can access these extra mailboxes using mail clients or the Roundcube Web-based mail program. The only differences are: you need to use the e-mail addresses as the username when connecting; and cloud-based calendars and addressbooks are not available.
This video shows you how you can create a new mailbox under a domain that you control using DirectAdmin, and how to connect to it using Roundcube.