What's a catch-all email account?
A catch-all email account is an address that is specified to receive all messages that are addressed to an incorrect email address for a domain. For example, you have three email addresses set up for coolexample.com; email@example.com, firstname.lastname@example.org, and email@example.com, you can set up firstname.lastname@example.org as a catch-all email account. Then, any email messages sent to email@example.com (or any other invalid email address), are sent to the catch-all account (firstname.lastname@example.org).
You can set up one catch-all email account per domain name.
Catch-all accounts may receive excessive spam, as *@[domainname.com] is a much bigger target than individual email addresses. If your customer has a catch-all email address and complains about spam to the associated domain name, let them know why they're a bigger target.