There have been problems with programs that supply their own MAPI DLL since M$ came up with that API. Netscape was notorious for breaking Outlook 98 if you let it make itself the default e-mail client. Keeping/restoring the Outlook DLL is best. Everything else seems to be a lot less picky and will work with that DLL.
Given that it's an other M$ DLL that is causing you grief, you may be OK, but as it is bound to be an older one... Keep an eye out for crashes in Outlook, or any other program you send mail from. Most likely cause will be the replaced MAPI DLL.