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Good morning,
Currently, we are on Exchange 2016 CU 19 with the latest security update installed, I know the security update also came out for CU 20. When upgrading to CU 20 do I then need to install that CU update and then the security patch from April(KB5001779) even though I installed them on the server under CU19?
Thanks,
Mitchell
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Hi Mitchell.
Glad to hear your question has been answered!
Just to add a little, it is also clarified in these links:
FAQ for March 2021 Exchange Server Security Updates
Released: April 2021 Exchange Server Security Updates
Thank you yes, I applied cu20 on my first server in my dag with no issue though when installing from a elevated cmd prompt I got these results. 
Luckily this was a test environment, I will attempt a different approach tomorrow.
Enter maintenance install cu 20>reboot>install SU while still in maint > reboot and exit.
Yes, apply CU20, then apply the post CU20 security update:
https://www.microsoft.com/en-us/download/details.aspx?id=103002
Thank you much just making sure!
Sure thing! If you could, please mark as accepted so we can close this up. Thanks!