Deliverability
Hard Bounce
A permanent delivery failure, typically because the email address doesn't exist or the domain is invalid.
Definition
A hard bounce is a permanent email delivery failure that occurs when an email cannot and will never be delivered. Common causes include non-existent email addresses, invalid domains, or addresses that have been disabled. Hard bounces should be immediately removed from mailing lists as they directly damage sender reputation. A hard bounce rate above 2% is considered problematic.
Examples
550 5.1.1 User unknown550 Mailbox not found550 Domain not found
Pro Tip
Remove hard bounces immediately - never retry sending to them. Verify lists before sending to avoid hard bounces.