This article will help you to fix an issue with your mailbox set up on cPanel, not sending emails out.
1. Setup a mailbox with the users credentials on your local machine and replicate the error. If you are unable to send out email, perform the following:
1. Determine if the domain.tld exists in localdomains file
# grep -i domain.tld /etc/localdomains
2. If it does not, check and see if the domain.tld exists in the remotedomains file
# grep -i domain.tld /etc/remotedomains
3. If it exists in the remotedomains file, login to WHM and navigate to "Edit DNS Zone"
and search for the domain.tld in question under "Choose a Zone to Edit" and click Edit.
4. Scroll down and look for the following:
Email Routing for "********" (The domain is owned by "********").
Change the setting from "Automatically Detect Configuration: ******** (recommended)" to "Local Mail Exchanger" and click save as per the screenshot: