Does anyone know how do I set up the required authentification so that I don't annoying warnings in gMail that read:

This message may not have been sent by: whoever@gmail.com

I'm using the mail functionality of SS as outlined here:
http://doc.silverstripe.org/sapphire/en/topics/email

I've made a simple little "share this" function that send a custom HTML e-mail from my page, and I am using the sender's email as the $from. It works great, but in testing I see that warning in gMail and it really bugs! Has anyone else run into this, is there a solution?

Thank you!

Right, it's a fairly new feature but it is actually an annoyance to a lot of people. Say for instance you have an ecard service and you want to have the sender as the "from" address. It even is affecting mailing list subscribers who get messages to my gmail account, sent from other gmail users through a mailing list, they get the “message may have not been sent by” alert even when the mailing list is a google groups-hosted group. Apparently there is a way to set up an SPF record (no idea what that is) so your message will be displayed as "sender@gmail.com via yourdomain.com." and avoid the annoying message.