We recently migrated from Exchange 2003 to Exchange 2010. Minus a few configuration issues the migration was without any major issues. But a within a few months of the final mailbox move a few of our Mac users running Outlook 2011 started getting errors when accepting meeting invites. If they replied with either an acceptance, decline, or tentative the meeting would get added to their calendar but the following error would be generated by Outlook 2011:
HTTP error. The server cannot fulfill the request Error code: -18500
The response email would remain in the Outlook 2011 sent folder and the meeting organizer would never receive a response, so the affected user would show up as having not responded in the scheduling assistant of the organizer’s scheduling assistant. The same issue would occur in Outlook Web Access, but with a different HTTP related error. This problem wasn’t encountered by Outlook 2007 and 2010 users so we believed it had something to do with the web services in our Exchange 2010 setup since both OWA and Outlook 2011 rely on the same services. But the one thing that bothered me was that the issue wasn’t affecting all our Mac users running Outlook 2011, only 3. So I went over our Auto Discover, Web services, and SSL certificate settings but didn’t find anything that would cause this issue. Web searches on the error code and message didn’t point to anything worthwhile either.
At the same time I was testing Room Mailboxes in Exchange 2010. Our company has relied on Public folders to schedule out various conference rooms and I was hoping to introduce Room Mailboxes to alleviate the pain points our users were facing with the current method. So I created two test rooms and tried booking them, strangely enough they would never accept my invites. I double checked the booking auto attendant settings, change registry settings on my computer in Outlook 2010 to force auto booking, and played with the resource policy settings on the mailboxes with no positive results. When I logged into the room mailboxes all my invites were sitting in the inbox. Out of curiosity I tried via OWA and it worked, then I tried via Outlook 2011 on my Mac and that worked. At that point I figured there had to be something weird with my Outlook 2010 setup. My first thought went to our new VOIP phone system.
About 2 months before our Exchange 2010 migration we upgraded our 20 year old phone system with a VOIP Shoretel System. One of the nice features was a software component (Shoretel Communicator) that would allow us to access all the features of our desk phones through our computers. In addition it had multiple Outlook integration features, one of them being calendar integration that would automatically change your call handling (e.g. go straight to voicemail) if you were in a meeting. This option added two extra buttons to the new appointment form allowing me to set various calling handling features for each meeting I scheduled. I was starting to think that this was the culprit, so I uninstalled the calendar integration but the two extra buttons remained. I then tried uninstalling the client, rebuilding my Outlook profile, and then singing into another computer (we use roaming profiles, but my account doesn’t). But those two extra buttons remained in my new appointment form. Strangely enough these two extra buttons only appeared when creating a new appointment in my calendar folder, but not any other calendar folder I had access to. So appointments created from shared calendars did not produce the error when sent through Outlook 2010, only my appointments with the two extra buttons did.
The final smoking gun was when I schedule a meeting in OWA when one of the Mac users running Outlook 2011 who was suffering from the meeting invite error. This user was able to accept my meeting invite when they previously couldn’t, so I tired sending this user another meeting invite via Outlook 2010 and the error returned. I then contacted our Shoretel re-seller and after some searching of the Shoretel support forums we found a few posts that pointed us to a possible issue. The Shoretel calendar integration replaces the default calendar form (IPM.Appointment) with a new one (Shoretel Appointment). This form has some added settings that the Shoretel system uses to set the users call handling, but this form also has some changes that Outlook 2011, OWA, and resource mailboxes don’t like. So we went around to each Windows user and changed the calendar form back to the default one and we are currently awaiting a resolution from Shoretel.
Looking back at the issue I think that in our environment this form was stored in each user’s Personal Form Library which is a hidden item in the root folder of the user’s mailbox (KB290802). This would explain why the issue followed my account even though I didn’t have a roaming profile. I assume this is the default location when a Organizational Forms Library isn’t present.