Hi, @BP-7667
Welcome back to the forum!
I'd like to offer some advice for you planning to migrate from EOP 2016 to EOL and install and configure Exchange 2019 on your persistent servers:
- You can install only the minimum roles required for management. In order to maintain a modern hybrid management server EOP, the roles you need include mailbox roles and CAS, which are critical for managing mail flow and client connectivity. You can refer to step 8 in this article Install Exchange Mailbox servers using the Setup wizard | Microsoft Learn
- On a permanent server, you need to first do a full installation with a hybrid configuration. Once a temporary Exchange 2019 server is decommissioned, while you can't selectively remove roles, you can disable or retire features that are not needed for management after the migration is stable, an approach that ensures that all necessary components are in place during the transition.
- If the server does not have an open port, a third-party certificate is not required, but it is generally recommended to use a third-party certificate for secure communication and administrative purposes. Even if the server is not exposed, having a third-party certificate can help avoid potential problems in internal communication and management tasks.
- If all mailboxes have been migrated to Exchange Online and no clients are connected to the on-premises Exchange server, you don't need to maintain split DNS or internal auto-discovery records that point to the on-premises server. However, autodiscovery is still used for some features in hybrid environments. We recommend that you point your autodiscover DNS records to Exchange Online to ensure that clients are connected correctly.
- For detailed steps on the minimum installation of Exchange 2019, you can refer to the Exchange 2019 Build Guide. The guide provides detailed steps on how to configure the necessary roles and services Exchange Server 2019 system requirements, Exchange 2019 Requirements, Exchange 2019 Memory Requirements, Exchange 2019 Client Compatibility | Microsoft Learn
- If you decide that you no longer need your on-premises Exchange server after a full migration to Exchange Online, you may consider retiring it. However, with Entra Connect Sync, recipient management becomes more complex without an on-premises Exchange server. You can try to manage it with the Exchange recipient management tool without the need for a local full Exchange server Manage recipients in Exchange Server 2019 Hybrid environments | Microsoft Learn
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".