Postmark :: InvalidMessageError when trying to send mail

I am using Rails 3 + Postmark-Rails.

First I created an account on postmarkapp.com to get my api key.

I activated my signature by email [for example, " vitalyp@dot.com "]

- After that I added this to application.rb

config.action_mailer.delivery_method   = :postmark
config.action_mailer.postmark_settings = { :api_key => "fdcb..." }

I use this method to send emails:

class UserMailer < ActionMailer::Base
  default :from => "vitalyp@dot.com"

  def do_my_mail(to, subj)
    mail(:to => to, :subject => subj)
  end

I get this error:

Postmark::InvalidMessageError
Sender signature not defined for From address.

Any ideas?