my mail is being sent to Google's spam folder

I've just set up my webserver, rails app, with postfix, and dovecot. When I'm testing the mailer it works fine, but Gmail is automatically putting my sent mail into it's spam folder.

I checked the Spamhaus list for my IP address and nothing, I checked the mail log, and there is nothing surprising there ...

can anyone offer me guidance for what else I should be looking for?

Thanks!

update

here is the mail header:

Delivered-To: test@example.com
Received: by 10.52.113.230 with SMTP id jb6cs31694vdb;
        Sat, 18 Jun 2011 05:42:32 -0700 (PDT)
Received: by 10.227.198.133 with SMTP id eo5mr3183668wbb.38.1308400951575;
        Sat, 18 Jun 2011 05:42:31 -0700 (PDT)
Return-Path: 
Received: from mail.example.com (mail.example.com [123.45.678.910])
        by mx.google.com with ESMTP id fs8si5078709wbb.140.2011.06.18.05.42.29;
        Sat, 18 Jun 2011 05:42:30 开发者_开发问答-0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of request@example.com designates 123.45.678.910 as permitted sender) client-ip=123.45.678.910;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of request@example.com designates 123.45.678.910 as permitted sender) smtp.mail=request@example.com
Received: from example.com (localhost.localdomain [127.0.0.1])
    (Authenticated sender: request@example.com)
    by mail.example.com (Postfix) with ESMTPA id CCB602AFCF
    for ; Sat, 18 Jun 2011 14:42:29 +0200 (CEST)

The first thing I would check is that your :from value in the email is the same of the :user_name in config.action_mailer.smtp_settings

Here's an example, notice 'youremail@gmail.com'

# config/development.rb
config.action_mailer.smtp_settings = {
    :address              => "smtp.gmail.com",
    :port                 => 587,
    :domain               => 'gmail.com',
    :user_name            => 'youremail@gmail.com',
    :password             => SECRET[:email_password],
    :authentication       => 'plain'
    # :enable_starttls_auto => true  
  }  

# app/mailers/user_mailer.rb
class UserMailer < ActionMailer::Base

  default :from => "youremail@gmail.com"  

  def welcome_email(user)
    @user = user
    mail(:to => user.profile.email,
         :subject => "Registration Successful")
  end
end

You may need to use custom domain. Do you? In theory anyone can set up custom mail server and send spam, and probably that's why it happens.

Also, be sure to not put anything too outlandish into headers.

i see that your email didnt had DKIM that can be also one reason of putting email in spam or maybe due to ip reputation or domain reputaion.

You can check if the ip and domain reputaion for blacklist here