4.4.2Bad connection

An outbound connection was established, but the message transaction could not be completed over it. The standard points to a timeout or inadequate connection quality. The code belongs to the temporary class, so delivery may be retried in a controlled way.

The X.4.2 pattern means that the outbound connection was established but could not complete the message transaction because of a timeout or inadequate connection quality. Under the standard, this detail is useful only as a persistent transient failure.

The leading digit 4 denotes a persistent transient failure: the current attempt failed, but the condition may clear. The code alone does not establish whether the cause was a timeout or connection quality, and it is not a permanent rejection of the address or message.

Class

Temporary failure

Retry

Controlled retry

Suppression

Check the full context

Operational recommendation: check suppression state before every attempt, then retry with backoff, jitter, idempotency, and a time or attempt limit. Stop after success, a permanent result, or exhaustion of the limit; refer repeated 4.4.2 results for diagnosis instead of retrying indefinitely.

Operational recommendation: do not add the address to a suppression list based on temporary code 4.4.2 alone. Inspect the full response, provider context, bounded-retry history, and other delivery events; suppress only after an independent permanent signal or when the applicable policy requires it.

• The message transaction did not finish before the timeout even though the connection had already been established.
• The established connection had inadequate quality for completing the message transaction.
• In the accepted Microsoft Exchange Server–Exchange Online hybrid TLS example, a SocketError is reported in a deprecated-TLS context; this is a provider-specific case, not the general definition of 4.4.2.

1. Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.4.2 and that the basic reply is in the 4xx class.
2. Correlate the response with the intended message, attempt time, and both connection endpoints; retain the full response and confirm in the logs that the connection was established but the message transaction did not complete.
3. Compare logs and timestamps from both sides for a timeout or signs of inadequate connection quality, without assigning either cause from the code alone.
4. If the response exactly matches the Microsoft example, confirm the Exchange Server–Exchange Online hybrid TLS flow and include the documented deprecated-TLS context; then compare bounded-retry outcomes and stop after success, a permanent result, or exhaustion of the limit.

An accepted Tier-1 example for Microsoft Exchange Server–Exchange Online hybrid TLS mail flow reads exactly: "421 4.4.2 Connection dropped due to SocketError". It documents a specific Microsoft practice — an asynchronous DSN with a 421 reply, code 4.4.2, and the SocketError subcode in a deprecated-TLS context — but does not change the standard meaning of X.4.2. No other accepted, exact provider example is assigned to this record.

421 4.4.2 Connection dropped due to SocketError

Compare 4.4.1, where a connection attempt received no answer; 4.4.3, which concerns an unavailable directory server; and 4.4.5, which concerns mail-system congestion. These distinctions help separate an incomplete transaction over an established connection from other temporary network and routing problems.

• 4.4.1No answer from remote host
• 4.4.3Directory server failure
• 4.4.5Mail system congestion

The canonical T0 meaning of X.4.2 and its class assignment were verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248 as of July 17, 2026. The exact provider-practice example comes from accepted Microsoft record p03-b03-microsoft-x-4-2-001; retry, diagnostic, and suppression guidance is presented separately as operational advice.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.4.2

• rfc5248T0 source

  Section 2.1: registry fields and non-exclusive Associated Basic Status Code

• rfc2034T0 source

  Section 4: enhanced status class agrees with SMTP reply class

• rfc3463T0 source

  IANA registry reference for X.4.2

• p03-b03-microsoft-x-4-2-001T1 source

  Symptoms section: exact NDR/error shown for deprecated-TLS mail flow between Exchange Server and Microsoft 365

Last verified: 17 July 2026