Hey - look what I found on one of my old machines. It's the documentation from the Exchange 5.5 SDK / EDK!
http://stephengriffin.members.winisp.net/55Edk/Exchange_55_SDK_Docs.exe
We're not supporting development against 5.5 anymore, and a rather large portion of what's in these docs only applies to Exchange 5.5, but there are some interesting tidbits still. Some highlights:
Note - these are old .CHM files. If you can't read them I can't really help you. I'm only providing them here as a historical curiosity.
BTW - for some reason, we've never pulled the 5.5 samples and libraries. You can still get those from here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=36a309c3-8c55-4476-8785-cafc59a2d075&DisplayLang=en
Thanks for making these available! I've been looking for these ever since the online version was pulled from the MS web site.
I promised I'd talk about accessing the Recovery Storage Group (RSG). The RSG is a neat trick we allow
I've given this solution to a couple customers so far and it appears to be working for them, so I thought
This post was inspired by a case I worked recently. In this case, the customer was using the 5.5 Event
There was a time when you could, in fact, use a script debugger with Exchange Event Service agent scripts. 
There are several options for configuring restrictions on MAPI client access to Exchange 2003 SP 2. …some
We have received several support calls in the past months relating to migrating Exchange Event Service Scripts from Exchange 5.5 to Exchange 2003. Because it isn’t straightforward or documented (to my knowledge), I came up with this information to help
Well all good things must come to an end, on December 31, 2005 Exchange 5.5 reached the end of its lifecycle. 
Suppose you’ve gotten a stream from WrapCompressedRTFStream and want to read what’s in it. Your code
Thank you very much! I am maintaining an old project which contain some old interfaces such as IExchangeManageStore. It seems those interfaces are not fashion but can work correctly.
It is very difficult to find function descriptions about those interfaces on the web. Why can't MS put all those old documents on the web to save our time?
Hey Does Exchange Server 2007 support Incremental Change Synchronization protocol ? Can you please tell me what are the Exchange server versions supporting this protocol ?
Yes and All