Here are some details from folks with more technical expertise in the area than I have, and it is specific to the iOS app:

Did you know that if you use Outlook on your iPad, Microsoft can read all your incoming email—independent of your device—and read all outgoing mail sent from your device?

Regardless of who your email provider is, Outlook transmits your credentials off-site to Microsoft servers and funnels connections through Microsoft servers on their way to your provider's server. We call that a man-in-the-middle attack.

and

MS says "To provide the best protection for your account, Safelinks are on by default. You can turn them off by signing in to https://outlook.live.com. Then select Settings > Premium > Security. There's a toggle under Advanced Security that you can use to turn off Safelinks."

See this MS Support page for more information.

https://support.microsoft.com/.../advanced-outlook-com...

It isn't apparent to the average user what is really happening. I quote a reliable source who says "Wow. So they're hijacking and tracking millions of links from private emails. Some of those sites are not indexed for good reasons. And when outlook.com has an outage, the links don't work. This is almost as bad as the credential stealing and man-in-the-middle attack that Outlook Mobile does. I wouldn't touch Outlook with a ten-foot pole these days."

Is your email confidentiality important to you? Consider changing your email provider from outlook.com to someone with a little more consideration for your privacy. I can see why more people are turning from MS applications to Open Office and other vendors.