Hard Bounce vs Soft Bounce: What's the Difference?

A hard bounce is a permanent delivery failure - the receiving server has rejected the message outright, typically because the address doesn't exist, the domain is invalid, or the mailbox provider has blocked the sender. Retrying does nothing; the address will keep failing.

A soft bounce is a temporary delivery failure - the address is valid, but the message couldn't be delivered on that attempt, usually because the recipient's inbox is full, the mail server is temporarily unavailable, or the message exceeded a size limit. Most email platforms retry a soft bounce automatically for a period before giving up.

The two need different responses. A hard bounce should result in the address being suppressed from future sends immediately - most email service providers do this automatically - because continuing to mail invalid addresses is one of the clearest negative signals mailbox providers use to judge sender reputation, and a rising hard-bounce rate can get a sending domain flagged or blocked well before any spam complaint does. A soft bounce is treated more leniently since the address may well be valid, but if the same address keeps soft-bouncing across several consecutive sends - a mailbox that's been full or inactive for months, for example - most platforms will eventually reclassify and suppress it too, since a soft bounce that never resolves behaves like a hard one for deliverability purposes.