I'm using Outlook 2016 with GpgOL 2.4.4 (Gpg4win 3.1.11) on Windows Server 2016 Standard (Terminal Server) and with Exchange 2013 on Windows Server 2012 Standard.

I have already asked about the following issue with encrypted eMails in the forum and it looks like I'm not alone:
https://wald.intevation.org/forum/message.php?msg_id=6897

Now I have tested it again and it behaves a little bit different than I had written in the forum before, but anyway very similar:

I have found out that the following options are not working in Outlook if applied to encrypted eMails:

E-Mail -> Right mouse click -> Categorize -> ...
E-Mail -> Follow up -> ...

Previous test results with Gpg4Win 3.1.10 (may be wrong):

For example, when selecting "Categorize...->Red category" for an encrypted eMail, then a red line appears in the detail view
and you may think it is OK. But nothing changes in the list view. Than, when switching to another eMail and than back to the just
categorized eMail, then the red line does not appear anymore und also no error message appears.

Test results from 10.01.2020 with Gpg4Win 3.1.10:

For example, when selecting "Categorize...->Red category" for an encrypted eMail, the category gets set, you can see it in the list view and in the detail view.
Or if you do "Follow up", the eMail gets marked and you can see it in the list view and in the detail view.
But you cannot undo it, you cannot remove the category and remove "Follow up", if you do it - nothing happens.
But it works with unencrypted eMails.

Test results from 13.01.2020 with Gpg4Win 3.1.11:

It looks like it was before 10.01.2020.

This is when using Outlook 2016 with Exchnage 2013.
At another PC, where Outlook 2016 installed on Windows 10 and is used without Exchange but with IMAP, there is no "Categorize...", but there is "Follow up", and there it works with encrypted eMails (if you do it by Right-mouse-klick in the list view and the eMail is not selected).