Why am I unable to send emails to a certain domain?

1. Check the recipient’s email address for any typos.
2. Check the email filtering settings or any email blocking rules on your domain.
3. Check if the emails are blacklisted or if the domain you are sending emails to has disabled emails being received from outside.
4. Try connecting with the email provider and verify if the domain is still using their service.
5. Check the incoming and outgoing port configurations of your SMTP server.