I also had trouble sending emails from outdated email nodes, but updating Node-RED to v0.17.4 fixed them for me. After upgrading I've been using a gmail account with "less secure apps" enabled, and in the email node I have "Use secure connection" ticked, with the default port and server.
I haven't had it working with an account that has two-factor authentication, though, so I'd recommend making a separate account just for Node-RED to use that is less secure apps enabled and only single-factor password authentication.
With those settings and the update it has been working great for me. I hope that helps!
To update your Node-RED instance log into manage.groov.com and put in your activation key (you can find this in groov Build, under Configure -> Licensing) to get the list of update files. You'll want the most recent Node-RED for groov box update, which right now, for v0.17.4, is version 4:
Once you've got that file go to groov Admin, choose to upgrade Node-RED from the quick start menu, give it the upgrade file, restart your Node-RED instance from Node-RED Admin, and you should be good to go!