When you using webmail feature in cPanel as image below,
you may getting error as below.
The E-Mail account of "cpanel_username@domain_name.com" is not exist, hence server could not verify the account.
1.) Once click at the Webmail option in cPanel, select on "Go to Webmail Login" -> Select one of the webmail application, eg. SquirrelMail -> Click on Options -> Personal Information.
2.) Type in a valid E-Mail account at E-Mail Address filed.
1.) Create the E-Mail account from E-Mails Accounts under Mail Option in the cPanel.
2.) Login to the webmail through URL as format below.
http://domain.com/webmail OR http://domain.com:2095