class ActionMailer::Preview

def call(email, params = {})

as they would if the mail was actually being delivered.
interceptors will be informed so that they can transform the message
Returns the mail object for the given email name. The registered preview 
def call(email, params = {})
  preview = new(params)
  message = preview.public_send(email)
  inform_preview_interceptors(message)
  message
end