Hi there,
we currently sign up two domain email backup service, we have some questions about the setup of our current DNS mx record:

1. we had add MX1.DYNU.COM and MX2.DYNU.COM to our mx record. as our original server with Priority 0, we add mx1.dynu.com as 5 and mx2dynu.com as 10, is it correct?

2. we put another domain with primary mx record original with Priority 10, then we add mx1.dynu.com as 15 and mx2.dynu.com as 20, is it correct?

thanks
eric

What you have described are correct settings. The lower the number, the higher the priority. Your setup ensures that your own email servers have higher priority than the backup Dynu servers.