To access your mailbox through Webmail:
Open a web browser, and type the URL: http://webmail.your-domain.com, where your-domain.com is the name of your domain. Press ENTER. A login screen will open. Type the full email address and password that you specified during creation of your mailbox and click Log in.
Normally, webmail is enabled, but it is possible for the account owner to disable webmail per domain name. To enable and disable webmail, follow these steps.
1)Login to Plesk and go to the correct domain.
2)Click Mail Accounts > Mail Settings.
3)Select the checkbox for "Activate mail service on domain" to enable it. Uncheck the box to disable it.