1. Check the server logs to see if the messages are being sent out from your server
2. Confirm that the other organization or email server is not blocking your emails
3. Check the Exchange anti-spam and virus protection policies and make sure they are not blocking messages
4. Request a trace of the emails and confirm that they are being accepted by the receiving server
5. Review the Exchange mail queues for any active issues
6. Check each user’s Outlook profile and confirm the correct server and port addresses are being used
7. Test the email flow by sending from internal and external accounts
8. Confirm the email settings for each user are set properly