It might not be rational any more, but I have serious trust issues with trusting an email program or a web browser from Microsoft. It dates back to the days of Windows NT, 2000, and XP, when Outlook allowed executable code to be attached to an email message, and automatically be executed when they message was ran, and when Internet Explorer served as the vector through which Windows system updates were downloaded and installed.
Outlook, of course, gave rise to a whole new category of malware,
“email viruses” that would infect a machine just by being foolish enough to use Outlook to receive and read a message containing it, and which would then go on to hijack the machine into emailing it out to others.
And think about how Windows Update used to work. You'd use Internet Explorer to visit the web site at
http://windowsupdate.microsoft.com/ and through that web site, you'd be guided through a process of determining which updates you need, downloading those updates, and then, from within the browser, installing them.
Think about what they means. A web browser, the very same web browser that many people were using for general web surfing, and probably visiting some fairly dodgy sites, had the ability and authority to install software that changed the underlying operating system. As easily as it could download and install legitimate update to Windows, it could just as easily be downloading and installing all manner of malicious crap.
I think it's fair to say that from the late 1990s to the early 2000s, Microsoft did more than any other entity in existence, to enable and empower creators of malicious software.