hi,
The idea of enabling the verify the authentication of sender in Lab tab in gmail settings is said to block receiving the non authenticate emails , the email with ?red question mark at the senders address? Is it advisable to enable it?. I enabled it as a measure to not receive any spam or hackers message from some of the sites, for which i have become a member, like say, a matrimony site. Recently i received an alert to change the password for the site, as somebody could steal the personal information , and says he is while hacker. I just did nothing. Till now, nothing happens. But receiving this kind of hack alert from ? mark emails , i think is a security lapse. I searched and found this
https://support.google.com/mail/answer/3070163?hl=en&ref_topic=3394218