Take a look at Backgroundrb (http://backgroundrb.rubyforge.org/) as a way to schedule a task (e.g. the sending of the mail itself) or the asynchronous plugin (http://code.inklingmarkets.com/asynchronous-rails-plugin/).
Both rely on a cron job to execute the pending jobs so maybe it is not what you are looking for if the email has to be sent right away.
Cheers, Atha