The latest version of Microsoft Exchange is still 2019, with supporting Cumulative Updates (CUs) released quarterly by Microsoft.

To move to Office 365 there are two typical approaches:

Cutover Migration

For this approach the user identities would typically be migrated to M365 Azure AD, and all mail data manually copied to Exchange Online – from the local client, or using the AZCOPY PST migration tool.

Hybrid Migration (Recommended)

This is the most common approach and allows for Exchange 2019 to coexist with Office 365 for address book, mail flow, and identity. The on-premise AD user remains the master identity, and the Microsoft Hybrid Connectivity Wizard (HCW) is used to connect Exchange 2019 to Exchange Online. Care is needed over an appropriate SSL certificate, and DNS changes. However, once achieved, the mailbox moves can be done in a phased manner, and mailboxes moved back easily also.

There are other considerations around SMTP relay requirements, and DKIM/DMARC also.

Exchange On-Premise Management

An on-premise Exchange 2019 server is required on-premise to support user management of email attributes however, with Microsoft only supporting this being removed if you want to use powershell instead.