Hello again, just wanted to share some knowledge. After reading some books and online sources I found out that what most spammers do these days is using free web host sites that offer smtp services, they then use a anonymous mass mailer php script to send their emails. As the emails are coming from a non blackliste, non private server, the email service provider doesn't recognize this as spam.
As for the gmail authentication, it is a very clever system that warns the user when a @gmail sender email is coming from a server other than gmail. Unfortunately it only authenticates @gmail emails, so any other address isn't recognize as fake.
The best thing to do is to check the headers when you are suspicious of an email, or never click links to websites, go to them through google, avoiding typos that come from writing manually in the address bar.
Hope this helps =)