Copy following dlls from Exchange setup media or Exchange server and place them in your \Windows\System32 directory.
Files starting with “net” are placed in Exchange server’s \system32 directory and others in \Exchsrvr\bin directory
After you have copied the files run
regsvr32 %systemroot%\system32\maildsmx.dll in the command prompt. Note that you need administrative rights so you need to start cmd.exe with “Run as administrator” selection.