How to diagnose "550 5.1.1 User doesn't exist" with forwarder

cPanel users can create an Email Forwarder for an email address that does not already exist within their account. For example, if John is the administrator of a site, he may want to tell users to contact him at admin@domain.tld so that he does not have to disclose his personal email address. Instead of creating a new email account, he can simply create a forwarder as outlined in our documentation.

Creating this forwarder allows him to receive mail addressed to admin@domain.tld within his john@domain.tld account, without requiring the creation of an additional email account.

Occasionally when configuring a forwarder in this manner, users sending email to admin@domain.tld might report that they receive a bounceback message with the following content:

LMTP error after RCPT TO:<admin@domain.tld>: 550 5.1.1 <admin@domain.tld> 
User doesn't exist: admin@domain.tld

Despite this, John will notice that the message was forwarded successfully and he received it in his inbox at john@domain.tld.

This problem occurs when admin@domain.tld did exist in the cPanel account previously, but not removed properly. admin@domain.tld will not appear on the Email Accounts page in cPanel, but it exists in the cPanel account's mail directory:

/home/user/mail/domain.tld/admin/

To prevent the "550 5.1.1 User doesn't exist" bounceback message, this directory can be removed or renamed, for example:

mv -v /home/user/mail/domain.tld/admin /home/user/mail/domain.tld/admin-bak