Why can't the Mail block see my variable?

I'm new to Ruby and wondering why I am getting an error in this situation using the 'mail' gem in a simple Sinatra app:

post "/email/send" do

  @recipient = params[:email]

  Mail.deliver do 
    to @recipient # throws error as this is undefined
    from 'server@domain.com'
    subject 'testing sendmail'
    body 'testing sendmail'
  end

  erb :email_sent

end

This however works fine:

post "/email/send" do

  Mail.deliver do 
    to 'me@domain.com'
    from 'server@domain.com'
    subject 'testing sendmail'
    body 'testing sendmail'
  end

  erb :email_sent

end

I suspect this is something to do with block scope and my misunderstanding of it.