Office 365 Dynamic distribution groups and on prem AD accounts

[Jujja]

Hi,

I have created a dynamic dist grp in exchange online.

We also have a previous on prem AD with sync on.

Its a hybrid solution.

Apparently some users doesn't receive the mail i send to the DDG.

I can see the the users not receiving email was created in the on prem AD years ago.

what have i missed?

[#2]

Where are their mailboxes located? Have you confirmed they are returned as actual members of the DDG? Have you run a message trace and what does it show?