When sending a message to valid recipients using Outlook, your message may be bounced back to you with the following error description:

Delivery has failed to these recipients or distribution lists:

The recipient's email address was not found in the recipient's email system. Microsoft Exchange
will not try to redeliver this message for you. Please check the email address and try resending
this message, or provide the following diagnostic text to your system administrator.

Diagnostic information for administrators:

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

The following reasons can cause this issue:

  • Outlook autocomplete nickname cache has an incorrect record for this recipient. To verify whether the nickname cache is causing the issue, try to send the same message to the same recipient using Outlook Web Access. If the message goes through, you need to clear the nickname autocompletion cache (read the Knowledge Base article on How To Reset The Outlook Nickname Autocompletion Cache for more information).
  • You replied to a message imported from another mail system that had an internal reply-to address. The resolution is to type the email address manually.
  • The Outlook contact is corrupt. Please check the address listed for the contact and correct it, or re-create this contact.