You can add an external mailbox hosted by a different provider (e.g: Gmail, Outlook.com, etc.) in your webmail interface.
1. You first need to know what are the the IMAP or POP and the SMTP servers as well as the port numbers and encryption methods to use. Please contact your mail service provider to receive this information.
2. Once you have this information you need to log in into your webmail account:
The username is the complete email address, for example: firstname.lastname@example.org.
The password is the one you created when you activated this mailbox.
Click on "Sign in".
3. Go to the left panel and click on "Add mail account".
4. Click on Manual.
5. In the "Add email account" window, you need to fill in the fields:
for IMAP or POP and SMTP.
6. Click on Save.