Connected to Exchange 2010
Users are receiving meeting requests for resources they are delegates for. They're unable to send a response out to the meeting request. When user attempts, the item gets stuck in the Outbox. Users receive the Error code: -18500. Error: HTTP error. The server cannot fufill the request. Mail could not be sent. The item will remain in the Outbox unless you delete the item. Users can accept the same meeting without problem in Outlook 2010 on PC or OWA.
I found a similar question asked before with the same scenarios:
- Meeting invite sent to affected Outlook 2011 user on Exchange 2010 from another user either through Outlook 2007/2010/2011,OWA,or ActiveSync Device
- Affected user accepts (with or without comments) or declines meeting invite
- Meeting gets added (or not if declined) to the affected users calendar and denial/acceptance email is queued locally in Outlook 2011
- Affected user gets the following error : HTTP error. The server cannon fulfill the request. Mail could not be sent. Account Name "EXCHANGE" Error code: -18500
- Acceptance/denail never makes it to the user sending the invite.
http://social.technet.microsoft.com/Forums/en-US/exchangesvrclients/thread/231711fb-9dbf-488e-9d78-dc776cbe3def
Note: Shortel VOIP is not installed on any of the computers. Message class for the meeting request is IPM.Schedule.Meeting.Request. Meeting was originally scheduled with resource through OWA and a second test was scheduled through Outlook on PC. Outlook for Mac user is still unable to accept meeting.