Outlook

4.3.2

Outlook is not accepting mail on this connection right now. Retry with backoff. Escalate only if it keeps repeating.

4.7.3

Outlook says the recipient org's queue quota is full. Wait and retry. The domain should accept mail again once backlog clears.

4.3.2

Outlook hit a temporary server error. Retry with backoff. No list changes needed unless it becomes constant.

4.4.0

Outlook could not replicate the message to a healthy secondary server. Retry later. This is usually on their side.

4.7.0

Outlook returned a temporary server error on the next hop. Retry with backoff.

4.7.650

Outlook rate-limited your sending IP for poor reputation. Check Microsoft SNDS, fix authentication, and slow volume until it recovers.

4.7.651

Outlook rate-limited your sending IP for poor reputation. Check Microsoft SNDS, fix authentication, and slow volume until it recovers.

4.3.1

Outlook is out of system resources on the receiving side. Retry later. Their admins need to free capacity.

5.1.3

Outlook rejected the address as invalid. Fix the syntax, then resend. Remove the address if it stays malformed.

5.5.0

Outlook says the mailbox is unavailable. Verify the address. Remove it if it stays undeliverable.

5.7.1

Outlook blocked the MailFrom domain on Spamhaus. Delist with Spamhaus, then wait for caches to refresh before retrying.

5.7.1-2

Outlook blocked part of your provider network. Slow volume and work with your ESP or ISP on the block list entry until it expires.

5.4.1

Outlook denied access for all recipients on this host. Ask the recipient admin to allowlist your sender if the mail is legitimate.

5.2.122

This Outlook recipient hit the hourly receive limit from all senders. Wait and retry later at a lower rate to that mailbox.

5.2.121

You hit the hourly send limit to this Outlook recipient. Wait and retry later. Spread traffic if many messages target one inbox.