As @Gary advised in another eM Client thread recently, when AirSync is used “the SMTP Settings are no longer used” as per the below extract from the bottom of the following eM Client blog page.
Quote from the above eM Client blog link page:-
“We have created a new update with a more direct workaround - the default Outlook.com setup now uses AirSync protocol to send emails instead of SMTP protocol.
So the account can be set up with the latest OAuth and still send messages”.
…
So you can disable the SMTP check box “under the General tab” and Save accounts, and you can still send mail as in my own example MS Personal Outlook account example below.
So if you are not able to send mail and have AirSync enabled, then “there could be a problem currently at the server end” or there could be a problem with the account itself somewhere in eM Client. You might to “try removing and re-adding the account” via the “automatic account wizard” in “Menu / Accounts”.
Lastly suggest to also try the latest V10.0.3351 update via the release history page if you haven’t already. If you do update, make a manual backup first via “Menu / Backup” incase you need to restore for any reason. Also close eM Client before updating.