> I also contribute to FOSS projects, and using Outlook is an impediment to projects whose mailing lists are based on inline posting

Based on my testing, that's not the only problem with using MS email clients on FOSS mailing lists. There's no concept of threading beyond the conversation view, and the client also mangles the email (wrapping or even sending base64 encoded test instead of the raw text. Even if your client sets the Message-ID header, MS servers will delete the header and replace it with their own.