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 receive 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
