Nope, Online mode is only an option for Exchange email account in Outlook desktop client. For IMAP account, it is by design that it would cache emails on your web server to local datafile and we cannot change this behavior.
For your needs, seems using web mail is a more suitable workaround. Or, would you mind trying Exchange email account?
Old question - new problems (Letsencrypt certs this time):
"Verify return code: 21 (unable to verify the first certificate)"
(meaning the Intermediate Certificate (chain) is not being found/used to verify)
Here is the solution I found after much trial and error over the last 24 hours.
First let me preface what I used for testing:
I have Dovecot on ...