I'm currently working on an Exchange Hybrid setup where all our mailboxes are in Exchange Online (EXO), but we still have one on-premises mailbox along with about 1000 distribution lists and mail contacts. I created a script to help with the migration: it creates identical staged M365 objects with a prefix added to their names and aliases, and it removes that prefix after we delete the on-prem objects from the Azure Synced OU. I also renamed the on-prem object with a similar prefix.
We believed that after renaming everything in M365, all we needed to do was change the domain setting from Authoritative to Internal Relay. The idea was that any emails sent from the on-prem mailbox would fail to find a local recipient and instead be routed to M365, but to our surprise, that didn't work at 1 AM! The logs showed no attempts to use the hybrid Send connector. During testing, I knew we could create an on-prem mail contact with the original primary SMTP address and the mail.onmicrosoft.com address for proper routing, but I didn't think that was the only method. Is there something I might be overlooking? We're stuck with these 1000 mail contacts or having to migrate the last mailbox, which isn't feasible right now due to its high email volume.
1 Answer
It sounds like it might be worth considering transitioning everything over to Exchange Online and just setting up a simple on-prem SMTP service for that heavier sending account. It could simplify things in the long run!
Are you suggesting something like SendGrid for SMTP? We were planning that too, but our initial migration strategy was to handle the distribution lists first, and now it feels like we've got to flip the plan upside down!