Outlook Error Codes

How to Fix Exchange Error 5.1.1 Bad Destination Mailbox Address in Outlook

Maira Mehtab Sep 27, 2026 869 views

Question details

The user needs to resolve an Exchange 5.1.1 Non-Delivery Report (NDR) indicating an invalid or non-existent destination mailbox address.

Product: Microsoft Outlook / Exchange
Device & OS: not provided
Scenario: Sending an email message to an internal or external recipient, often using an automatically suggested email address from the Outlook cache.
Observed behavior: The email fails to deliver, bouncing back with an Exchange 5.1.1 error code stating the recipient address is invalid or incorrect.

Before you start

Ensure you have the exact spelling of the recipient's email address and confirm with them through an alternative method if their email address has recently changed.

Solution 1Recommended

Clear the Outlook Autocomplete Cache for the Recipient

This is the most effective solution for internal recipients whose routing addresses may have changed, causing the cached entry to become invalid.

Outlook remembers the email addresses you've used previously to save time. However, if an internal colleague's account was migrated or recreated, the hidden routing address tied to their name might be outdated, triggering the 5.1.1 error.

Create a new email

Open Microsoft Outlook and click on 'New Email' to open the composition window.

Trigger the autocomplete suggestion

Begin typing the recipient's name or email address in the 'To' field until the autocomplete drop-down list appears.

Delete the cached address

Use your arrow keys to highlight the incorrect suggestion, or hover over it with your mouse, and click the 'X' icon on the right side (or press the Delete key) to remove it.

Manually enter the address

Type the full, correct email address manually into the 'To' field and send the message again.

Cache Cleared: Once you successfully send an email to the manually typed address, Outlook will save the new, correct routing information to your autocomplete list.

Solution 2

Verify Email Delivery Using Outlook on the Web

Testing email delivery through the web client helps determine whether the 5.1.1 error is caused by your local desktop application or a server-wide issue.

Solution 3

Review the Non-Delivery Report and Contact IT

If clearing the cache and using the web client do not work, the issue may stem from Exchange Server configuration or an expired mailbox.

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Frequently Asked Questions

What does Exchange error 5.1.1 mean?

Exchange error 5.1.1 indicates that the destination email server cannot find the recipient's mailbox. This typically happens if the email address contains a typo, the mailbox has been deleted, or your email client is using an outdated autocomplete suggestion.

Why am I getting a 5.1.1 error for a colleague inside my organization?

If an internal colleague recently had their account recreated or migrated to a different server environment, their underlying Exchange routing address (LegacyExchangeDN) might have changed. Your Outlook client may still be attempting to use the old routing address stored in its autocomplete cache.

Will the recipient be notified that I tried to send them an email?

No. A 5.1.1 Non-Delivery Report (NDR) means the message was rejected by the server before it reached any inbox. The intended recipient will not know you attempted to contact them.

How can I prevent the Bad Destination Mailbox Address error in the future?

To minimize these errors, manually type the email address when messaging a new contact, regularly delete outdated suggestions from your autocomplete list by clicking the 'X' next to their name, and keep your Outlook address book synchronized with your organization's global address list.

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