The account or domain may not exist, they may be blacklisted, or missing the proper dns entries. (DNS Entries)

Symptoms

When sending mail to a domain on your server, the message fails with the following error:

CONFIG_TEXT: user@example.com is undeliverable: The mail server could not deliver mail to user@example.com. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries.

Cause

This issue can occur if the receiving domain is missing the proper MX records to resolve the intended mail server.

Resolution

The domain's MX records will need to be checked to ensure they are setup correctly. If the cPanel server is your DNS host, you can see the records in the DNS Zone Manager:

How to edit a DNS record in the DNS Zone Manager

An MX record must point to a fully qualified domain name (FQDN) and will look something like the following:

CONFIG_TEXT: example.com. 3359 IN MX 10 mail.example.com.

Note: If the domain has its DNS hosted externally, you will need to review this further at the domain's DNS host. 
How to find out where a domain's DNS is hosted

Additional Information

The account or domain may not exist, they may be blacklisted, or missing the proper dns entries. (Mail Routing)

How to create an MX record in cPanel