Disable or Turn Off S/MIME for everyone

@Jonathanjo The "defaults read" line doesn't set the DisableSMIMECompose value, it reads it. You'd use "defaults write..." to set the value.

That said, instead of writing a script to set that flag, which would not prevent your users from changing it, you should use a Configuration Profile with an Applications & Custom Settings payload for the com.microsoft.Outlook domain with the following .plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>DisableSMIMECompose</key>
    <true/>
  </dict>
</plist>

Ahh I didn't noticed that 'read' instead of 'write'. Thanks @sdagley for correcting me.

I'll try this out way.

Is there any way to disable 'Digital Signing' too? 

@Jonathanjo You can see the different options for Outlook at https://docs.microsoft.com/en-us/deployoffice/mac/preferences-outlook

The easiest way to configure those in Jamf Pro is via a Configuration profile using the following steps:

1. Create a new Configuration Profile
2. Configure the General Settings for the profile (Name, Level: Computer Level, Distribution Method: Install Automatically)
3. Select the Applications & Custom Settings item in the payload list, then select External Applications (Source: Jamf Repository, Application Domain: com.microsoft.Outlook, Version: 16.57, Variant: Microsoft Outlook.json)
4. The default for the Preference Domain Properties section is to not show all options, so click the Add/Remove Properties button to see the possible options.
5. The Disable Microsoft 365 encryption options item may be the one you're looking for.