kinda a tough question that might have a solution using HTML in gmail (or any other provier)
assuming you have sent an email from your server , the email is HTML formatted , user receives the email to his SPAM folder , when he reads the email , a portion of the message tells him that "This message has been displayed as spam" or "mark this message as not spam" , but if he receives the message to the inbox , this text is not displayed .
been trying to do it using image alt text , kinda failed :( .
Either include at the bottom of each email "Please add user@example.com to your address book if this message has been marked as spam" or to use a 1x1px transparent image with ALT text reading the same. From a usability point of view, it's not really a problem to ask every user to whitelist your address, those who didn't have it marked as spam will ignore.