The answer: 1300 (not 1301, we mean 1300)
While debugging our insertion technology for migrations from legacy calendaring systems into Exchange 2007, Russ decided to insert 23,000 appointments into a single calendar (which is not as rare a real-world occurrence as you might think).
When we narrowed th einsertion to just recurring items we found a hard limit at 1300:
We were really surprised to be getting CDO errors server-side, especially since CDOEXM is obsoleted and CDOEX is denigrated in 2007, and our new insertion code is based on EWS and has had all CDO-specific code ripped out of it with the diligence of a vegan removing meat and dairy products from their diet.
The specific Outlook error message: "There is not enough memory available to perform the operation." led us to some server-side errors that looked like this:
Exception type: Microsoft.Mapi.MapiExceptionNotEnoughMemory
Exception message: MapiExceptionNotEnoughMemory:
Unable to save changes. (hr=0x8007000e, ec=-2147024882) Diagnostic context
Event Type: Error
Event Source: EXCDO
Event Category: General
Event ID: 8199
Calendaring agent failed in message save notification with error 0x8007000e on firstname.lastname@example.org: /Calendar/test5.EML
We didn't find anything on specific 2007 problems, but this does sound very similar to an existing 2003 problem: You cannot save or delete a recurring appointment from a Calendar that is maintained on an Exchange Server 2003 computer, aka KB 892524.
So we wanted to put this up to let folks know and see if anyone else had been experiencing this problem.