Hello kbrown730,
Thank you for posting this case in the Microsoft Community!
Based on your description: With Microsoft's default font change to Aptos - and with them saying it will be the default in Outlook as well - will this font be an EMAIL SAFE FONT? I would like to cooperate with you to work on this case. To clarify this case, currently, the default font is Aptos matter which is used in Outlook for Windows or Outlook on the web. Original article and guide published April 2019. Nov 2019 updates: 14 new typeface families with a total of 85 fonts added. February 2021 updates: 5 new typeface families, additional weights for existing families. **Warning:**new Bahnschrift fonts are not supported in PDF. See note in the guide.Jan 2022: New script fonts, display fonts, and Congenial font family. July 2022: new Aptos font family and ADLaM Display. For more information, refer to this article from Microsoft: Cloud fonts in Office. You can check the cloud font list as shared in the above article which Aptos font is safe and available.
Thus this default font Aptos is safe in Outlook. If you want to change the default font in Outlook, you may check the following article as a reference:
Change font size and appearance of email in Outlook- Microsoft Support
Change the font or font size in the message list - Microsoft Support
Hope the above workaround will be helpful. If you need further assistance, please feel free to let me know, and I will keep assisting you.
Your understanding and patience will be appreciated.
Thank you!