If you get reports from users that their emails are stuck in either the Drafts or Sent Items box and are not being sent, you should run the Microsoft Exchange Troubleshooting Assistant. If you get the following error message from the assistant, check the free space of the drive that the Exchange Transport Role is installed on to ensure it has at least 2.5GB of free space.
Once you free up space, go to SERVICES.MSC and restart the Microsoft Exchange Transport Service. Email should start working again.
Note: If you receive this error on a SBS 2011 box, one of the best ways to free up space is to move the WSUS database to another drive. Instructions for how to do this for SBS 2011 can be found here.