I recently had a client that simply could not replay to an e-mail she received.  She had no issues sending new message or replying to any other messages, but when trying to reply to this one she would get an error saying “Outlook can not send item”. 

I found an article stating that is was because of a e-mail address that was in one of the previous messages before she received it.  The quick and easy workaround was to click reply on the message and click “Format Text” and choose “Rich Text, or Plain Text” and then send the message.  Apparently Outlook doesn’t like messages that contain a e-mail addresses that look something like this; mhtml:{d89988d-dc34-56n2-mki1-156audu5u7ac}mid://099878999/! x-stw:mailto:myusername@mydomain.com.  While this doesn’t give a good reason why this happens it will make it possible for them to get their mail out as needed.