This one drove me nuts for a while, I have an smtp server running on my server, but occasionally, the SMTP administration snapin would disappear for no particular reason. This is needless to say VERY annoying - as there's not other effective way to administer SMTP and the snapin doesn't appear in the normal list. Well, forget where I found the solution but here's the fix - just run this command -
regsvr32 c:\windows\system32\inetsrv\smtpsnap.dll and you'll magically see it's back!