If you are getting a lot of undeliverable messages, it is important to first check with your email provider to find out the source of the issue. Then,

1. Check that the address is correct. Before sending an email, make sure you double check the intended recipient's address.
2. Check for typos. If you mistype a single letter, the email may not be delivered to the correct recipient.
3. Check your email settings. Make sure that your email client is configured properly to send email.
4. Check your email for suspicious messages. Some email programs will help protect you from suspicious emails.

These are all great ways to help ensure that your emails are being delivered properly.