Group policy for Outlook 2016 to set default font

You would have to use Group Policy Preferences in GPO.  

All those settings are stored in the same location: HKEY_CURRENT_USER\Software\Microsoft\Office\<Version#>\Common\MailSettings

If you don't have any legacy OS clients prior to WIndows 7 the the GPE (PolicyMaker former name from 2006) is part of Win 7/2008R2 forward.  GPP will work we just need to make sure those settings are set to Enforce and not replace and we are going to have to create collections, convert the .reg to XML, import, and few other things.  

We are still looking at the same Options for Stationary relative to Outlook and same registry keys.  Perhaps they shifted those to GPP which is still GPO but uses slightly different engine down level.

It's definitely more work relative to just having these settings in the Outlook ADMX file under Stationary but the settings just are not there.  Have to use GPP.

Basically, your going to have to logon to each workstation that has the different version of Outlook.  I state this because you might have different versions of Outlook so my solution allows for having different versions. 

Your going to have to Open Outlook for each different version but in your case let's say it is just Outlook 2016 clients so you only have to do this one time.

Open Outlook, then Outlook File tab click Options > Mail > Stationary and Fonts
Click on Font and set for New mail messages and Replying or forwarding messages and any other settings. 

Then, your going to export this key to .reg 
HKEY_CURRENT_USER\Software\Microsoft\Office\<Version#>\Common\MailSettings

Now, we don't necessarily want to tag everyone's registry with every value in this reg file so I would open the file in Notepad++ and remove everything except (for example)

(This should cover the settings you need)

Value name: “ComposeFontComplex”
Value name: “ComposeFontSimple”
Value name: “CReplyFontComplex”
Value name: “ReplyFontSimple”
Value name: “TextFontComplex”
Value name: “TextFontSimple”

Then save the file.  Important to use Notepad++ to assure the file is saved in proper format otherwise the XML convert might fail.  The key here is to make sure it is saved in the original format by making sure the file type is *.* for example if your using regular notepad instead.  Can't be saved as text formatted.

You cannot import .Reg to GPP so you will need to convert this first to XML.  Otherwise, you have to open each binary value from the registry and mimic that in GPP settings so converting to XML is much easier and you can do this easily with PowerShell.  

I looked at this script and it is from reputable source so download the script:
https://4sysops.com/wp-content/uploads/2014/09/RegToXML.zip

Extract the zip, copy somewhere like C:\Posh.  
Copy the .reg file you exported to C:\Posh
Open Powershell as Administrator.  CD to C:\Posh in Powershell window.

Now
      
.\RegToXML.ps1 c:\POSH\Outlook2016.reg c:\Posh\Outlook2016.xml

This is where it get's a little tricky.  

You have to right-click the XML file in File Explorer and then copy it to the Windows clipboard. (not open it)

Now, in the Group Policy Editor, navigate to 
User Configuration (or Computer Configuration) > Preferences > Windows Settings > Registry 
Right-click the Registry symbol, navigate to New, and then select Collection Item > Outlook2016
Click on the Outlook2016 Collection
Right-click the white pane on the right. In the context menu, select Paste.
Confirm the import.
Make sure the settings are set to Enforced not Replace. 

Now, this is going to tag the registry for everyone but they would simply have all these HKCU settings for all Office Versions but Office is only going to reference the one with the correct version in the path.

Now, repeat all the steps for each version of Outlook.

More work but this should achieve the desired result.

Once you have converted all the .reg files to .xml and imported to the different collections - each Outlook version - you might not see the settings down level initially.

Simply logon to the workstation or if in existing session....make sure Outlook is closed.  Open CMD prompt...Type:
gpupdate

gpupdate /force is not required, that downloads all policies and waste network bandwidth.  Gpupdate in-of-itself will download the deltas.

Hope this helps.