What's the difference between hard bounces and soft bounces?
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When you send an email and it doesn't reach the recipient, your ESP gets back a message from the receiving server explaining why. That message determines whether it's classified as a hard bounce or a soft bounce.
A hard bounce is permanent. The address doesn't exist, the domain has expired, or the mailbox was deleted. There's no point retrying. Your ESP should suppress hard-bounced addresses immediately, and if it doesn't, do it manually. Hard bounces are the ones that actually damage your sender reputation if you keep accumulating them.
A soft bounce is temporary. The server accepted the connection but couldn't deliver the message right now. Maybe the inbox is full (452 error), the server is temporarily overloaded, or the receiving domain is rate-limiting your IP. Most ESPs retry soft bounces for 24-72 hours. If they keep failing, some ESPs eventually convert them to hard bounces after a threshold of repeated failures.
The practical difference: hard bounces require immediate action, soft bounces usually don't. But watch for addresses that soft bounce repeatedly over weeks. That pattern often means the mailbox is abandoned or the person is blocking your domain. Consistent soft bouncers belong on a suppression list just like hard bounces. If you're trying to understand what SMTP error codes like 550 or 421 actually mean, there's a full breakdown here.
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