What are the most common causes of hard bounces?

Hard bounces result from permanent conditions that will not change:

Address does not exist:

User never existed at this domain. User account was deleted. Typo created invalid address.

Domain issues:

Domain does not exist. Domain has no MX records. Domain not configured for email.

Permanent account status:

Account permanently disabled. Account converted to alias without forwarding. Mailbox abandoned and removed.

Policy-based permanent rejection:

Address or domain permanently blocklisted. Sender permanently banned. Content type never allowed.

Response to hard bounces:

Suppress immediately. Do not retry. Remove from active lists. Investigate source for list quality issues.

Hard bounces are closed doors. The destination simply does not exist or will never accept from you.