External emails via O365 to on-prem DL issues

[Affinity100]

Our MX are pointed to O365 about 2 weeks ago so far all looks good.

However, today I noticed when an email from external send@xyz.com is sent to a DL which is on-prem exchange 2010.  the on-prem 2 users are not able to receive that email.  Right now 2 users have been identified including me.  There could be more.  This DL has about 130 users that are o365 hosted and on-prem.

Message tracking don't show my email address or the message was delivered to me.  There is one server which is receiving emails from O365 or the receive connector is setup to receive emails from O365.

Any thoughts where do I start?  Thank you

[#2]

Check if the on-premises DL's are configured to receive unauthenticated email. Can you run this in PowerShell?
get-distributiongroup "name of group" | fl name,RequireSenderAuthenticationEnabled
If it is set to false try:
set-distributiongroup "name of group" -RequireSenderAuthenticationEnabled $true