Clear Outlook auto-complete cache (RESOLVER.ADR.ExRecipNotFound error)

In case your customer gets an NDR with the error similar to the one in the screenshot below, the most probable reason for it is that the email address is corrupted in the Outlook auto-complete cache. (This situation often happens, if the recipient mailbox has been recently migrated.)

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound;
Recipient not found by Exchange Legacy encapsulated email address lookup'

And the email address listed starts with "IMCEAEX-"

Solution

1. Open a new email in Outlook and start typing the problematic email address in the To  field.

2. When it appears, click on the X next to the address to delete it from the Outlook auto-complete cache.

3. Type the address in manually and try the sending again.

Background

Outlook stores internal email addresses in a humanly barely readable way, which is stored on the exchange server in the so called Exchange Legacy DN attribute (the strange looking address highlighted in blue in the screenshot starting with IMCEAEX-). If this stored address becomes corrupted in the Outlook cache, the server won't be able to find the address Outlook was referring to when the email was sent.

When the auto-complete cache is cleared, and the address is typed in again manually, Outlook will recreate this cache based on the newly looked up value that the server returns as the Exchange Legacy DN and the delivery will be successful.

Most commonly, the problem is caused by a (pst) migration. To circumvent this issue, before the migration, it's advisable to add the Exchange Legacy DN as an X500 address to the migrated mailbox in the new mail server.