How do ESPs classify bounces?

ESPs classify bounces to guide appropriate handling:

Hard bounces:

Permanent delivery failures. Address should be suppressed. Examples: User not found, domain does not exist, invalid address format.

Soft bounces:

Temporary delivery failures. Retry is appropriate. Examples: Mailbox full, server temporarily unavailable, rate limited.

Block bounces:

Rejected due to reputation or policy. May be temporary or permanent depending on cause. Examples: Blocklist rejection, spam content filtering, policy violation.

Technical bounces:

Infrastructure issues. Often temporary. Examples: DNS failure, connection timeout, TLS negotiation error.

Classification methods:

SMTP response codes (5xx vs 4xx). Enhanced status codes (X.Y.Z). Message text pattern matching. Sending domain and known behavior patterns.

Classification determines response. Suppressing hard bounces protects reputation; retrying soft bounces recovers messages.