1. Check the configured routing settings for your mail server and make sure that it’s correctly set up to route emails to the desired recipient.
2. Check to make sure that the address that you’re sending to is valid.
3. Verify that you, as the sender, are using the correct email address to send the emails.
4. Ensure that the recipient hasn’t been blocked from receiving emails from your address.
5. Make sure that you’re not accidentally copying in incorrect recipients while composing a new message.
6. Double check to make sure that the recipient hasn’t changed their email address recently, and if they have, update your address book accordingly.