[Aelius]

Maybe I've misunderstood something, but if gmail didn't do this, what's to stop me from sending emails that look like they come from your address?

[leephillips]

Nothing. The standard allows you to put anything you want in the From: header. Sometimes I send emails from Santa Claus or God. The envelope information is still there.

[leephillips]

Actually, not "nothing." If the MSA doesn't like this, it is free to reject the email. But not to change its content.