Hi, probably it’s a spam filter on that domain, you should ask them to check the spam folder to see if, at least, the email is received. If not, it is probably dropped. There is not much you can do if they have your domain or sending IP in their blacklist.
You can try to use an SMTP service if your provider gives you the mailboxes for your domain or a delivery service like SendGrid, Amazon SES, Mailgun, …
See more here: https://www.thenewsletterplugin.com/documentation/troubleshooting/