550 5.7.1 RESOLVER.RST.AuthRequired; authentication required

[GeorgeShorty]

I work with a hybrid environment where almost all of the user accounts are now 365, but there are still some distribution lists that have not migrated yet.  I have a particular on-prem Exchange 2010 dynamic distribution list that will not allow a 365 remote mailbox user to send to it.  They are a third user, and the other two (also 365 remote mailbox users) can send to it just fine.  Their accounts are all on the same domain as the dynamic distro, yet the third user (recently added to authorized senders list) cannot send.  All users authenticate to the domain with the same tools, yet my third user cannot send to this dynamic distro.  The "Require that all senders are authenticated" is checked and hasn't been an issue.  Given the nature of the distro list that authentication requirement remains in place.

What am I missing?  What do I need to check/test?

v/r,

George

[#2]

Hi,
have you checked this?
https://theitbros.com/exchange-550-5-7-1-resolver-rst-authrequired-authentication-required/

[#3]

I did check that, and it doesn't apply - they are talking about an email outside the org sending to inside the org, the resolution to which is to remove the check from requiring authentication. That is not my scenario.

My case is a user inside the org (remote mailbox user) trying to send to a distro list that is also inside the org (on-prem dynamic distro). Two other people who are also inside the org (both remote mailbox users) can successfully send to the distro, but my third user cannot. Given the nature of the distro list, the stipulation for authenticating senders is still required.

[#4]

mailboxes in exchange online are in a different organization than recipients in your exchange onprem.

[#5]

That doesn't change the fact that two of the three can send to the distro, while the one cannot. I don't see any differences in the three accounts, but maybe I'm not looking in the right places?