Switching from Internal Relay to Authoritative With Sub-Domains

Question

I am working on trying to switch my Exchange hybrid from Internal Relay to Authoritative, and have a few questions that I am hoping someone can provide guidance. I have a sub-domain, that is tied to Google Mail (GSuite). Having Exchange set to Internal Relay allows me to have the option to Accept Mail for All Subdomains. If I switch over, I understand that I lose that option. One thing I did notice in my current setup for Internal Relay is that Accept mail from all subdomains is not checked. Currently all the users in the sub-domain are setup as Contacts/Mail Users in Exchange Online vs a standard mailbox and there are no mail delivery issues. What I am trying to figure out is the following before I make the switch: Will there be any impact at all with mail flow to the sub-domain since the option is not checked. Is having a Mail User sufficient as a recipient to ensure proper mail flow (I believe the answer is yes) If after switching to to Authoritative, I find there is failure in mail delivery, would there be any issues switching it back to Internal Relay.

Any guidance would be appreciated.

Answer 1

Hello @Prescimone, Chris - Admin,

Switching Exchange hybrid from internal relay to authoritative could really have an impact on mail flow, especially if your subdomain is tied to another mail service like Google Mail (GSuite). Considering your concerns:

1. Impact on mail flow to subdomains: Normally, when a domain is set to Authoritative, email is only delivered to valid recipients in the Exchange organization. With an internal relay domain, emails will be delivered to recipients that exist within the Exchange organization, while other emails will be relayed to another email server in a different location. Switching does have an impact, while you are in the current setup. If the "Accept mail from all subdomains" option is not selected and there are no mail delivery issues, it may indicate that your mail routing is configured correctly for the specific subdomain. Switching to authoritative mode may not affect this, and if you're not sure, you can test your mail flow by sending a test email before making the switch.
2. Mail user as recipient: Yes, having a mail user is usually enough to ensure proper mail flow.
3. Switch back to internal relay: If you have issues after switching to authoritative, you should be able to switch back to internal relay without major issues, but there may be some disruption to your mail flow.

Therefore, if you do need to switch, it is recommended that you test during non-working hours first to make sure everything is working as expected.

Hope the above information is helpful to you!

---

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.