If your emails are being rejected or bounced back from a certain sender, the first thing to do is check if you have spelt the recipient’s email address correctly and make sure you are using the correct domain.

Next, consider the following checklist to try to solve the problem:

1. Check your recipient's email account for a bounce-back message generated by their mail server
2. Check your sent mail folders and search for the sent email in question
3. Check your email server to see if the message was blocked for any reasons such as spam filtering or flooding
4. Verify that the recipient’s domain has not been blocked by your mail server
5. If you are sending messages in bulk, move to a more reliable email delivery service

Once you've done this, then it's likely you'll have identified the problem and be able to fix it.