Mails, passwords, calendars and contacts. Basically everything. Here's another blog article: https://mailbox.org/en/post/warning-new-outlook-sends-passwords-mails-and-other-data-to-microsoft

Sending the entire email content to their cloud isn't that good.

However an advantage to doing so is to be able to use push notifications on the app without having to poll continuously the email address from the device. Which in return reduces the battery usage compared to constant polling.

However, they could have done something like spark mail, only get the email subject, sender and a little bit of the content to put into the noficiation then delete after the push notificdation has been sent.

Can't even turn autosave on for Word docs anymore without letting Microsoft save your shit to the cloud.

See, Microsoft cares so much about you they'll even make a backup of all of your emails, completely for free, without you even having to ask. And here you are complaining...

I hate with the intensity of a million suns that they always have this absolute fucking bullshit argument "For better experience".