olemus
saadetise tagastus ajutistel põhjustel:
postkasti täitumisel, serveri ülekoormusel vms
= mail return for temporary reasons: the mailbox is full, the server is overloaded, etc

ülevaateid
https://sendgrid.com/blog/email-bounce-management/

https://en.wikipedia.org/wiki/Bounce_message

https://help.mailup.com/display/MUG/Types+of+bounces