I'm not sure if this is what you're talking about, or even what version of
Outlook you're using or what language, but Outlook caches items and releases
them when it feels like it. You can't force a release of an item from the
cache, in fact except in Outlook 2007 you can't even detect from Outlook
when the item is released.

There is a method in 2007 that tells you that, the item.Unload() event.

Using managed code (VB.NET or C#) can also contribute to that since even
setting an item to null won't release it fully until the RCW is destroyed
and the garbage collector runs on the item. You can help accelerate that by
calling Marshal.ReleaseComObject() on the item until the function returns 0,
then calling GC.Collect().