To diagnose why emails appear in the Outbox instead of sending, there are several steps to follow in order to find a resolution:

1. Check the Outbox to make sure the message isn’t stuck and waiting to be sent (or delete and recreate).
2. Check the email server settings are correct and have not been changed.
3. Temporarily turn the anti-virus off to make sure the message isn’t being held in quarantine.
4. Reconnect the Exchange server to the SMTP system.
5. Reinstall or configure the mail client.
6. Check that the client is supported by the mail server.