Are you getting the following error message when trying to send an email?
There was an error sending your message: Failed to add recipient: [SMTP: Invalid response code received from server (code: 550, response: Verification failed for No Such User Here Sender verify failed)]
If you are, it means that you are logging in to your webmail with your cPanel user name and password. You are logging into a generic account that is not actually configured to send email, and the server is not recognizing that the generic account even exists (which is why you are getting the error).
…this means that you are logging into webmail incorrectly.
When you log into your email or webmail, please make sure that your user name is the full email address you want to access and your password is the password created specifically for that email address.