A bounce is any non-delivery response from a receiving server, but not all bounces mean the same thing. Classification parses the SMTP response code and diagnostic text to distinguish a permanently invalid address from a temporary mailbox-full condition or a reputation-based block.
Misclassifying a soft bounce as hard (or vice versa) either suppresses a recipient who was only temporarily unreachable, or keeps retrying an address that will never accept mail, both of which damage reputation over time.
PulsePigeon classifies every bounce against the originating provider's response format and feeds the result directly into suppression-list decisions, not a generic regex over the SMTP text.