Expected response code 250/251/252 but got code "550"

When sending an email out, I get a number of “failed” emails with this message in the log: Expected response code 250/251/252 but got code “550”, with message "550 <email@gmail.com> No such user hereAny idea why that is… one of the email address is my personal account I seed into every email blast.

Running Mautic version is: 3.3.3