An "Out of Band" (OOB) bounce happens when an email is initially accepted by the receiving server but rejected later. Think of it like a letter being accepted at a post office but returned days later.

Out of Band bounces do not have a RFC standard, therefore we do not recieve the same details for an OOB as an in-band bounce - we are reliant on what the mailbox provider tells us. 

Common reasons for OOB bounces:

• The email address was found to be invalid

• The mailbox became full

• The message triggered spam filters after deeper analysis

• The recipient server's policies changed after initial acceptance