Microsoft has released security updates for vulnerabilities found in:
Exchange Server 2013
Exchange Server 2016
Exchange Server 2019
These updates are available for the following specific...
Updated May 04, 2021
Version 8.0
Blog Post
This from the FAQ seems conflicting to me:
My organization is in Hybrid mode with Exchange Online. Do I need to do anything?
While Exchange Online customers are already protected, the April 2021 security updates do need to be applied to your on-premises Exchange Server, even if it is used only for management purposes.
Do I need to install the updates on ‘Exchange Management Tools only’ workstations?
Servers or workstations running only Microsoft Exchange Management Tools (no Exchange services) do not need to apply these updates.
As I said before, I have an Exchange lite (Exchange Express) install because it's hybrid, literally it's just got management tools on it (no exchange services)