Symptoms
You may see an error like one of the following, where USER@EXAMPLE.COM is not an email address that the mail was sent to.
USER wasn't found at EXAMPLE.COM.
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.
Description
This can occur if one of the recipient addresses is forwarded to a non-existent email address.
Workaround
If the mail is being sent to one of the addresses on your server, you will want to check the forwarders for the recipients in the following location of cPanel and ensure they are correct.
Home > Email > Forwarders
If the mail is being sent to an email address hosted at another server, the owner of that email address will need to check this.