So I had this issue today where all emails that I wrote would be sent to the 'Drafts' folder instead of sending.  I was also unable to recieve new emails as well.  At first I thought this would be some kind of 'back pressure' issue and ignored it, but when it kept happening and the server still had 50GB of hard drive space left and 1GB of RAM available I figured that something else must be wrong.

Well, it turns out the 'Microsoft Exchange Transport' server stopped (still can't figure out why exactly).  So I started it up and everything is fine again. 

Just a heads up for anyone with an issue like this where back pressure is not the problem.