Update for rails 4.0
Now you need these code to make it work:

# I recommend using this line to show error
config.action_mailer.raise_delivery_errors = true

ActionMailer::Base.smtp_settings = {
  :address        =>  smtp.gmail.com ,
  :domain         =>  mail.google.com ,
  :port           => 587,
  :user_name      =>  foo@gmail.com ,
  :password       =>  ****** ,
  :authentication => :plain,
  :enable_starttls_auto => true
}

You can set up ActionMailer to use Gmail s SMTP server using something like this in config/environment.rb:

ActionMailer::Base.delivery_method = :smtp
ActionMailer::Base.server_settings = {
    :address        =>  smtp.gmail.com ,
    :domain         =>  <your domain> ,
    :port           => 587,
    :user_name      =>  <your gmail> ,
    :password       =>  <your password> ,
    :authentication => :plain
}

Edit: If you experience any difficulties, set your config to display errors:

ActionMailer::Base.raise_delivery_errors = true

Have a look at ActionMailer. In RAILS_ROOT/config/environment/ , there is a file for different environments (development, test, production) the configurable settings go in these files

You specify the delivery_method like this,

ActionMailer::Base.delivery_method = :sendmail

or if you want

ActionMailer::Base.delivery_method = :smtp

A detailed example of the settings has been posted by Mikael S

HTH

If I understand your situation correctly, you want to send an email from your local computer using a custom email address such as john@mycompany.com. If you already registered the domain name for your email account ( mycompany.com ) is very likely that the company that is hosting your website, also has a POP/SMTP server. If so, you can use Mikael S s sample and change the address parameter to your Hosting company s smtp address and use your hosting company s username/password.

If you have not register your custom domain or don t have a hosting provider, you can install a free email server in your local computer. If you use WindowsXP, you can add the IIS email server by going to add/remove programs->windows features. If you are using Linux, you can use any of the email servers available in the repositories. Once you install your local email server you will use Mikael S s sample code and use 127.0.0.1 or localhost in the address field. If you are using WindowsXP s email server, I think you don t have to enter username/password.

Hope it helps you.

You can send it from localhost, you can even set the sender as a real mailbox e.g. you@gmail.com.

However, some (or say most) servers will not accept this mail as part of their spam blocking strategy (inability to verify the sender identity). However, In the past, I have had something similar with python which worked on gmail.

so good luck ;-)