Using Mac OSX10.6.3 with Entourage 2004, I was getting a dialog box reporting error -17099, but could send and receive emails. Nevertheless, the box was a real annoyance. There are numerous posts regarding this error. I tried a number of solutions that were offered before noticing that many of them reported problems that were fixed by correcting errors in email addresses on outgoing messages. I looked in my outbox and found a message that I had placed there with no email address. I deleted that message. Problem fixed. It only took me four hours. I hope this helps somebody solve his -17099 problem more quickly.