If you find that your email fails to be sent to a particular domain, it is possible that the IP address of your mail server is blacklisted.
To verify if your IP is blacklisted, here are some steps you can do to check:
First, go to www.whatismyip.com to check what is your IP address. Copy down the IP address.
Next, go to mxtoolbox.com/blacklists.aspx, then enter your IP address at the Server IP box and click “Blacklist check” button.