Hello Jack
Do you have "from record owner" set to "yes" on the Email Send? If so, that means that the sender information for each individual email in the Email Send will dynamically be set to the name and email address of the user that owns that recipient's record; it does not send the email from the owner of the Email Send record. As such, a number of different users could receive out of office replies and auto responders since the email is set to send from a number of different users.