Bad connection — transient network problem
Production Risk
Low for isolated incidents; Medium if systematic — may indicate network or TLS configuration problems.
The connection between the sending and receiving mail servers was lost or degraded during the SMTP session. This is a transient failure and delivery should be retried.
- 1An intermittent network outage dropped the TCP connection mid-session.
- 2TLS negotiation failed due to a transient certificate or protocol error.
- 3The receiving server's connection limit was reached and the connection was reset.
The TCP connection is dropped or reset during an active SMTP session before the message is fully delivered.
# MTA log: lost connection with mail.example.com[192.0.2.1] while sending DATA # DSN: 421 4.4.2 Connection dropped
expected output
421 4.4.2 ...
Fix
Allow standard MTA retry logic to handle the failure
WHEN Receiving a 4.4.2 in MTA logs
# Verify Postfix queue and retry: postqueue -p postqueue -f # Force retry of all queued messages
Why this works
Bad connection errors are transient. Standard MTA queue management will retry until the connection succeeds or the message ages out.
✕ Treat 4.4.2 as a hard bounce
4.x.x codes are transient — the message should be retried, not discarded.
RFC 3463 — Enhanced Mail System Status Codes
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