Below you may find a list of known issues that appear after updating to Entourage 12.1.3. We would strongly recommend you to install the latest Entourage update to fix these issues:
Entourage 12.1.3 update known issues when sending or replying meeting requests:
1. Meeting invite responses may get stuck in outbox.
Any meeting replies (accept, tentative, decline) will not send. Additionaly, users may get an error "Proxy error. The Proxy cannot fulfill the request." Again this only happens on the meeting replies... all emails are sent fine.
Reference Link: http://www.officeformac.com/ProductForums/Entourage/6874
2. You cannot send a meeting invite that you have just created. Additionaly, you get an error "Unexpected data was encountered" with an error code -17997.
Please use one of the following workarounds:
1. If you have POP, IMAP account in the same Entourage identity, you may want to recreate your identity from scratch and creating the Exchange account(s) first, BEFORE any non-Exchange accounts.
2. Dragging the message from the outbox to the drafts folder and sending from there can be used as a temporary workaround. Only for sending invites.