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‘550 5.1.1 RESOLVER.ADR.ExRecipNotFound’ when sending email using Outlook in Office 365

Having recently carried out a migration from an on-premise Microsoft Exchange solution to Microsoft Office 365 we have had users receiving ‘550 5.1.1 RESOLVER.ADR.ExRecipNotFound’ non-delivery reports (NDRs). Users had all their email exported to PST files and then re-imported back in to their new mailboxes.

I have found and wanted to share the reasons and resolution to the problem because this happens quite a lot.

The cause in our scenario was down to the email migrated from a users PST file and then imported, this had carried across incorrect Outlook nickname cache (AutoComplete) entries.

The Fix

To resolve the problem you can either reset the nickname and the AutoComplete caches in Outlook or by deleting a single recipient when your replying to a migrated message.

Outlook 2010 or Outlook 2013

To reset the whole Outlook nickname cache for Outlook 2010, use one of the following methods:

Method 1

1. Open Outlook.
2. On the File tab, click Options.
3. Click the Mail tab.
4. Under Send Messages, click Empty Auto-Complete List.
5. Click Yes.

Outlook will generate a new nickname cache.

Method 2

1. Click Start, and then click Run.
2. Start Outlook by using the /CleanAutoCompleteCache switch. To do this, type Outlook.exe /CleanAutoCompleteCache

Please note: If Outlook is not installed in the default location, you must point to the path of Outlook.exe.

Outlook will generate a new nickname cache.

Mike Keats

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