With the merge today of [nemo-qml-plugin-email] Fallback to PLAIN auth when testing an account. by dcaliste · Pull Request #50 · sailfishos/nemo-qml-plugin-email · GitHub (and some other fixes), I’m now able to add a Mailbox mail account, just entering my email and password, with the automatic settings for in- and out-going servers working.
I’ve no idea if it will be part of 5.2 though…
Next step : implement the OAuth2 authentication method, like for the Google account.