MailFixIt

Outlook SMTP delivery response

Understand Outlook 550 5.1.1: Recipient not found

The receiving Microsoft system could not resolve the destination mailbox address used for this message.

Problem description

A representative response is:

550 5.1.1 Recipient not found.
Is this temporary or permanent?

This is permanent for the address as submitted. Correct or restore the recipient before sending again.

What this response means

550 5.1.1 Bad destination mailbox address means the receiving Microsoft system could not resolve the recipient used in the SMTP transaction. It is not an SPF, DKIM, or DMARC failure.

Fix it as the sender

  1. Confirm the complete address with the recipient through another channel.
  2. Create a new message and type the address manually. Do not resend or reply to the failed message.
  3. Remove the old suggestion from Outlook autocomplete, then select or enter the corrected address.
  4. If only this recipient fails, stop retrying until the address is confirmed.

Fix it as the recipient administrator

  1. Confirm that the mailbox, shared mailbox, group, mail user, or alias exists and is licensed or enabled as required.
  2. Check its primary and proxy SMTP addresses.
  3. If the object was moved or restored, verify directory synchronization and mail routing before testing again.
  4. Use message trace and the original NDR to confirm which address Exchange actually looked up.

What MailFixIt can check

Public DNS cannot prove whether a private recipient object exists. MailFixIt may help only if the full NDR also reports a separate domain or MX problem.

Use the related Microsoft guides for tenant routing issues, not for a simple nonexistent recipient.

Frequently asked questions

What does Outlook 550 5.1.1: Recipient not found mean?

The receiving system could not find the destination mailbox address; the address may be mistyped, removed, moved, or stale in the Outlook recipient cache.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

Outlook 550 5.7.515: Access denied, sending domain does not meet the required authentication levelDiagnose Outlook 550 5.7.515 email rejection safely. Check SPF, DKIM and DMARC DNS, interpret the receiver report, and identify the next configuration step. Outlook.com requirements for high-volume sendersCheck Outlook.com high-volume sender requirements for SPF, DKIM and DMARC alignment, then review sending-IP PTR as separate infrastructure evidence.

Primary sources: Microsoft 365 non-delivery reports.