When you send a message to a non-existing e-mail address, you will get an automatic reply saying that this e-mail address does not exist.
This is what we call a bounce e-mail and this message will be automatically sent to the e-mail address you specified as “bounce address” in the AcyMailing configuration page.
Depending on your server, this message may not be returned immediately and sometimes it can be sent up to one week after you sent your e-mail.