Thousands of new contact records created from CiviMail (Postfix/Gmail?) mis-configuration
You say: Should I just delete the "default" email-to-activity entry

AFAIk there is no such default. Looking in dmaster.demo.civicrm.org i do not see one

https://dmaster.demo.civicrm.org/civicrm/admin/mailSettings?reset=1

I don't know that you need to delete it but changing the email address it uses should stop the problem.