[red-iron-pine]

There is a regulatory body for CPAs, which include things like ethics. There are "generally accepted accounting practices" (GAAP) which are adopted by regulatory bodies like the SEC and IRS. There are laws that are related to all of the above.

Point is, my dealings with my accountant(s) is highly regulated, and there is expectations of confidentiality, trust, and privacy -- with rules and laws to back that up.

Not the case with some random app handling your corporate data. Plus Outlook and other calendars integrate with many other systems, and there are a lot of nasty hacks if you can get someone's email. Lotta risk for negligible gain.