I was getting a lot of incomplete SimpleTest results in the VM because Drupal couldn't send emails.

I added this to my Vagrant file, which installs sendmail with a shell script:

config.vm.provision :shell, :inline => "sudo apt-get install -y sendmail"

which kind of works with my Ubuntu 12.04 base box (after restarting the VM fix a network issue), but there must be a more Chef-ey way to do this.