How do ESPs categorize bounce types?

ESPs categorize bounces using multiple signals to guide appropriate handling:

Primary categories:

Hard bounces: Permanent failures requiring immediate suppression. User does not exist, domain invalid, permanent rejection.

Soft bounces: Temporary failures eligible for retry. Mailbox full, server busy, greylisting.

Block bounces: Reputation or policy rejections. May be temporary or permanent depending on cause.

Technical bounces: Infrastructure issues. DNS failures, connection errors, TLS problems.

Classification methods:

SMTP response codes (4xx vs 5xx). Enhanced status codes (X.Y.Z). Message text pattern matching. Provider-specific signature databases.

Accurate classification drives correct response. Suppress hard bounces; retry soft bounces; investigate blocks.