You have your mail client setup for your cpanel username which is an email account that is disabled by default because the cPanel username email account acts as a "catch-all" email address and that leads to problems for about 99% of users because it collects anything@yourdomain.com and essentially is a spam magnet.

Go into cPanel > Mail Icon > add/remove/manage accounts and add an actual email account. Then use this account for sending and receiving email.

Each email address you create will have a unique username of email@yourdomain.com and a password associated with that username.

Then you can either login to that account directly at yourdomain.com/webmail or you can login to the webmail interface with your cPanel username and password. Choosing the second option gives you a directory tree of all the actual email accounts you have created, and you can access them all from the common cPanel login.

Logging into yourdomain.com/webmail with one of the email addresses as the username allows you to access only the specified account, which is useful for sites that do not want to give all email users superuser access via webmail.

Logging in as the cPanel user to webmail is useful for personal sites that want access to each inbox on one common superuser interface.

Finally for outlook (or whatever mail program you use) you will need to configure each individual email address with the full email address as the username and that email address password, not your cpanel user and password.