How can I send mail with rails without a template?

The simplest way to send mail in rails 3 without a template is to call the mail method of ActionMailer::Base directly followed by the deliver method,

For ex, the following would send a plain text e-mail:

ActionMailer::Base.mail(
  from: "[email protected]",
  to: "[email protected]",
  subject: "test",
  body: "test"
).deliver

http://api.rubyonrails.org/classes/ActionMailer/Base.html#method-i-mail gives you all the header options and also ideas about the how to send a multipart/alternative email with text/plain and text/html parts directly.

Rails 5 users may find the accepted answer (using format.text {...}) doesn't work; at least I was getting an exception because Rails was looking for a view.

Turns out there's a section in the Rails Guide called Sending Emails without Template Renderingand all one needs to do is supply :content_type and :body options to mail(). E.g.:

class UserMailer < ApplicationMailer
  def welcome_email
    mail(to: params[:user].email,
         body: params[:email_body],
         content_type: "text/html",
         subject: "Already rendered!")
  end
end