A common question about Outlook 2016 for Mac is 'How can we change the default font size for incoming mail?'
Outlook for Office 365 Outlook 2019 Outlook 2016 Outlook 2013 More. Less Conditional formatting is a way to make the incoming messages that meet defined conditions stand out in the message list by using color, fonts, and styles.
It's obvious how to do this for outgoing mail (Preferences > Fonts), and it is easy to temporarily change the font size using CMD+ or CMD-, but there does not appear to be a way to permanently change the default font size for incoming mail via the GUI.
After speaking with Microsoft Technical Support, and doing a bit of research, I thought I would post a workaround here.
Warning: Follow this procedure at your own risk. You may want to back up any files before changing them.
1. Close Microsoft Outlook.
2. From Terminal.app, change to this folder:
/Users/<user_name>/Library/Containers/com.microsoft.Outlook/Data/Library/Preferences
Notice that this path is different, and longer, than the normal location of plist files. At this location, there should be a file called com.microsoft.Outlook.plist.
The full path to the plist file will probably be:
/Users/<user_name>/Library/Containers/com.microsoft.Outlook/Data/Library/Preferences/com.microsoft.plist
How Do I Make Text In Incoming Email In Outlook 2016 For Mac
Incoming Mail
3. From this folder, run this command:
sudo defaults write com.microsoft.Outlook PlainTextWebPrefsIdentifierWebKitDefaultFontSize -int <desired_font_size>
Type your password if prompted.
This will change the default font size for plain text. You can also change the font size for HTML messages:
sudo defaults write com.microsoft.Outlook HTMLWebPrefsIdentifierWebKitDefaultFontSize -int <desired_font_size>
Incoming Email Alerts
4. Restart Microsoft Outlook. The new font size should be apparent when reading incoming mail.
Additional information
Microsoft appears to be using binary plists. These differ from the usual XML-style plists that Mac users are used to. If you want to open the plist to edit with a text editor, you will first need to convert it to XML using the plutil command:
plutil -convert xml1 com.microsoft.Outlook.plist
This converts the plist to XML and allows you to edit it from a text editor. When you re-open Outlook, it should be converted back to the binary format. You can also convert it to binary, manually, by using plutil:
plutil -convert binary1 com.microsoft.Outlook.plist
Note: I am not a Microsoft employee. I don't know why Microsoft doesn't just include this functionality in the GUI to make it easy. But this will hopefully be helpful to people who have been searching for a workaround.