This is an issue with the Mac version currently.
https://forum.manager.io/t/cannot-email-with-custom-smtp-on-os-x/
I just retested with 16.4.20 and this still seems to be the case.
@lubos Do you have any updates on the updates?
This is also assuming that you’re not running Windows or Linux on the Macbook, in which case it would be something else.