Types of bounce messages
From ASSPSMTP
Hard bounce
A hard bounce is a type of Non-Delivery Report that indicates a mailbox delivery issue (the address or domain doesn't exist, etc.), a delivery failure due to administrative policy (intentional blocking, spam filtering, etc.), or a routing failure that did not succeed after extensive mail queuing. Hard bounces typically require administrative intervention on the recipient side.
Hard bounces are 5xx/5.x.x series status codes.
Typical hard bounce verbiage:
- Blocked
- Domain not found
- Failed
- Failure
- Nondeliverable
- Relay
- Returned
- UBE (unsolicited bulk email)
- UCE (unsolicited commercial email)
- Undeliverable
- Unknown user
- User unknown
- spam
Soft bounce
A soft bounce is a type of Non-Delivery Report that indicates a problem that is considered temporary (the mailbox is full/over-quota, server is busy/overloaded, message is too large, etc). Soft bounces typically can be corrected by user intervention on the sender-side or on recipient-side - depending on which side the problem actually exists.
Soft bounces can be 4xx/4.x.x or 5xx/5.x.x series status codes - depending whether or not the issue was something that could be automatically retried (queued) by the server, and if the queuing ultimately succeeded or failed.
Typical soft bounce verbiage:
- Connection refused
- Connection timed out
- Mailbox disk quote exceeded
- Mailbox full
- Mailbox is full
- Message exceeds size limit
- Recipient over-quota
- Too many hops
- Warning
