Centralized mail flow - Mailing lists issue - Hybrid Setup

Azzmoh Both options are correct 

1. Mail contact in Office 365 (Like a DG) pointing to On-Premise Address and hence gets delivered to Mailing List on your OP Servers (resolved and delivered to recipients within it - Even if the actual members are outside the Organization).

2. If you dont want to manually create or autmate creating of these Mailing lists in your Office 365, first make sure your Office 365 is internal relay and then disable the Centralized Mail Flow.

Best would be to Export these Lists into On-Premise Exchange/AD Distribution lists and sync them via Azure AD Connect to Office 365, so in that case no need to separately create contact in Office 365 or disabling the Centralized Mail Flow 🙂

Cheers !!

Ankit Shukla