Rencently I've discovered the reason why all the mail messages sent by my Web app were putted on spam folder when the "to" header ends with "@gmail.com".
You must include "Message-Id" header or your message will be treated as spam by Google.
This is the easy way to do this with System.Net.Mail.MailMessage Class:
Message.Headers.Add("Message-Id", String.Concat("<", Now.ToString("yyMMdd"), ".", Now.ToString("HHmmss"), "@yourdomain.com>" ))
And that's all (obviusly you must follow the Google recommendations too).
Wish to save at least the same time that I spent resolving this to many of you.
