Currently we are operating in HYBRID Configuration with centralized mail flow enabled and utilizing shared name space scenario.
On-Prem Exim Unix mail servers and exchange online i.e. O365.
All mailboxes are in Exchange online.
Current Mail flow :
Internet > On-Prem > O365
O365 > On-Prem > Internet.
We now want to switch from On-Prem to O365 by changing the MX records but there is one concern which is am not sure on how to get around it.
We have a Sympa Mailing List server which allows users to create mailing lists just like Distribution lists in Exhcange or Exchange online. Mailing lists resides in SYMPA ( on-Prem ) and are never synched to O365 as there is no provision for it.
So my first step is to come out of Centralized mail flow so that outbound from O365 is routed directly to internet and not touching my on-prem i.e. when an O365 mailbox users sends an email to internet then it goes out directly to internet without hitting on-prem server but my QUESTION is when an O365 mailbox sends an email to MAILING list ( SYMPA ) which doesnt have any entry in O365 surely it will end up in NDR as the mailing list resides on-Prem.
Assumptions to make it work :
If i create a mail contact for that mailing list in O365 will it send it out to on-prem using connector as when it searches within itself there is no entry for it ? Am i correct in assuming so ?
Secondly if i make my Accepted domian in exchange online as INTERNAL RELAY will it do the same as above ? i.e. when O365 mailbox sends an email to mailing list (which doesn't exists in exchange online but is present in on-prem ) it searches within itself and then routes that using connector to on-prem ? am i correct in assuming so?
If there are any other better solutions then let me know please.
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 :)
Best Response confirmed by
Azzmoh (New Contributor)