Where is your mail hosted currently? Have you made any changes to the MX record, verify that it's pointing to the correct system? If possible, send a test email from one such account and post the (sanitized) NDR message here.
Receiving Emails from Outside Recipients
I am unable to receive emails from outside recipients (yahoo, hotmail, gmail, etc.) to my email address. The other staff members with same email domain are also having same problem. We had this happen since changing out website (same domain but different host and builder) though our previous website was not synced with our outlook emails. We are able to send/receive emails internally and from other business accounts but not from main domains like yahoo, hotmail, gmail, etc. I checked the message flow options and it does allow all senders and no authentication is required. So not sure how to fix this