Here is a list of troubleshooting steps to try if you are unable to send emails to a certain reciepient in Exchange:

1. Check to make sure the address of the recipient is correct.
2. Ensure that you have a valid sender address.
3. Verify that the sender hasn’t been blocked by the recipient.
4. Check for any general Exchange server outages.
5. Confirm that your email is within limits.
6. Check for any issues with the recipient’s email domain.