Email Returned Undelivered

Thank you for testing, Brian. I saw your email come through. I would greatly appreciate it if you can shed some light on the following:
1. What would make a member's email return if they send the email from a registered email address to members-5456@toastmastersclubs.org?
2. A few members faced issues between December 15 and December 17. Was the server down at the time?
3. Which address is more appropriate to send emails to members-5456@toastmastersclubs.org or 5456.toastmastersclubs.org?

Don't have a reason, just have them resend the emails if they still need to.