Why does the email I send to a specific recipient from my Exchange mailbox fail?

If you receive a bounce-back email in your Exchange mailbox where the error message is similar to this:

Delivery has failed to these recipients or groups:

The email address you entered couldn’t be found. Check the address and try resending the message.

And you are able to email the recipient from your non-Exchange mailboxes, then this is more than likely due to cached contact details in your mailbox.

To remove the cached details:

1. Open a new email in your Outlook
2. In the ‘To’ field, enter in the name of the contact you are attempting to contact
3. When the auto-complete box appears, click on the X symbol next to the details
4. This will clear that address from the cache
5. Enter in the recipient’s email address and send an email to them
6. Further emails will now be sent successfully