After searching extensively online and on this site I haven't found an answer. I've seen other users that have 'Send As' permission issues but that is not entirely the case here...
We have an Exchange 2007 server running. Most of our users connect either via webmail or IMAPS/SMTP. Occasionally I've had a few user's report that they were unable to send mail. I'd check their 'Send As' permissions and sure enough 'NT AUTHORITY/SELF' was missing. I'd add it back in and all would be well.
What happened to my account yesterday made me think this was a symptom of an ongoing problem. I had that error happen to me went in and checked and sure enough me 'Send As' permissions didn't include 'NT AUTHORITY/SELF'. I added the permission back and went about my business. Two hours later the same thing happened and I had to re-add the permission. This morning it happened once again. My account had not locked out or otherwise been disabled though I don't think that should have any effect on the permissions anyway.
Does anyone have any idea why users would spontaneously have 'NT AUTHORITY/SELF' removed from their 'Send As' permissions?