Unable to test the GMail custom actions

Question

I have been trying to test GMail actions for a few days but it does not seem to work. Since I am not registered I use the little piece of Java code below to send an email fr

Answer 1

Even test emails need to be signed with DKIM/SPF in order to prevent spoofing, and I'm not sure if there's a way to do that with SMTP.

If you don't want to use Apps Script, a Google Apps domain (with proper DKIM/SPF configuration) is probably your best bet.