Clearing out ActionMailer::Base.deliveries after RSpec test

I have the following RSpec test for my UserMailer class:

require "spec_helper"

describe UserMailer do
  it "should send welcome emails" do
    ActionMailer::Base.deliveries.should be_empty
    user = Factory(:user)
    UserMailer.welcome_email(user).deliver
    ActionMailer::Base.deliveries.should_not be_empty
  end
end

This test passed the first time, but failed the second time I ran it. After doing a little bit of debugging, it appears that the 1st test added an item to the ActionMailer::Base.deliveries array and that item never got cleared out. That causes the first line in the test to fail since the array is not empty.

What's the best way to clear out the ActionMailer::Base.deliveries array after an RSpec test?