I just tried the Outlook desktop client at home for the first time with my Fastmail account. This screen [1] pops up during the Add Account process, saying that all my messages will be synced to the Microsoft Cloud for "better experience".
According to the linked support article, there are no options to disable this "feature" aside from removing the account. I used a new account to test it out, and was unable to find out where the data was synced to and there's no way to verify it was deleted completely after I removed the account.
To be fair the good MS people did not mention that your synced data will ever be deleted, they only mention that you can disable the sync (by removing the account) and surely once you have removed the account you won't be able to access anymore the already synced data, a good question would be what happens if you re-add the account, will earlier synced data be accessible again?
Regardless whether the data is deleted or not, I find troubling that this synchronization with the Microsoft cloud is forced on the user (if I get this right you cannot add an e-mail account to outlook without opting in for this synchronization).
According to the linked support article, there are no options to disable this "feature" aside from removing the account. I used a new account to test it out, and was unable to find out where the data was synced to and there's no way to verify it was deleted completely after I removed the account.
[1] https://postimg.cc/SXjMjWCL