Exchange NDR Lookup

Cause: The recipient's mailbox has exceeded its storage quota.

Fix: Recipient must free space or have their quota raised. Sender can retry later.

Cause: The receiving server is low on disk/resources.

Fix: Transient — usually resolves on retry. Check destination server health.

Cause: Message could not be delivered within the retry window (often downstream queueing or DNS).

Fix: Check recipient server availability and MX/DNS. Often resolves once the remote host recovers.

Cause: Sending is being throttled (often suspected spam / compromised account).

Fix: Check the account for compromise; review outbound volume; may need to wait or open a support case.

Cause: Unspecified permanent delivery failure from the remote server.

Fix: Read the full NDR text for the remote diagnostic; verify the recipient address.

Cause: A problem with the sender address (often a bad reply-to or distribution group setting).

Fix: Verify the sender address and any forwarding/alias configuration.

Cause: The recipient email address could not be found at the destination.

Fix: Verify spelling; confirm the mailbox/alias exists and is licensed.

Cause: Recipient address has no Microsoft 365 / on-prem mailbox, or directory sync issue.

Fix: Confirm the mailbox exists, is licensed, and that mail-enabled object / MX points correctly.

Cause: Recipient mailbox is full and the server will not retry.

Fix: Recipient must clear space; sender should resend afterwards.

Cause: The message exceeds the recipient or connector size limit.

Fix: Reduce attachment size or raise the message size limit on the connector/policy.

Cause: Destination refused to accept mail for the recipient, or no route exists.

Fix: Check accepted domains, connectors and DNS MX; confirm the recipient domain accepts your mail.

Cause: Mail is looping between servers (often a bad forwarding rule).

Fix: Remove conflicting forwarding/transport rules causing the loop.

Cause: Sender is not permitted to send to this recipient — relay denied, blocked, or policy.

Fix: Check transport rules, connector scoping, and recipient restrictions; verify SPF/auth if external.

Cause: The recipient requires the sender to authenticate / be a tenant member.

Fix: Recipient-side policy — the recipient admin must allow external senders.

Cause: The sending account is disabled or blocked from sending.

Fix: Re-enable the account / clear the sending restriction.

Cause: The sender domain's SPF record does not authorize the sending IP.

Fix: Fix the sending domain's SPF record to include the correct sending source.

Cause: Message failed DMARC / was not authenticated and the recipient rejects such mail.

Fix: Ensure SPF and DKIM pass and DMARC aligns for the sending domain.

Cause: Outbound spam / rate limiting on the sending tenant.

Fix: Investigate possible account compromise; review outbound spam policy.

Cause: The sending IP is on the recipient organisation block list.

Fix: Recipient admin must allow the IP, or send from a non-blocked source.