install smtp server features through server manager if its windows server machine or through add windows features from 'add/remove programs' dialog option
even if you are on iis 7 or anything higher, open up iis 6 manager and make sure smtp virtual server is running (specially if you are getting error message saying 'Remote machine actively refused connection)
if you get 5.7.1 relay error - check the properties of smtp virtual server , general tab ..instead of all 'All UnAssigned' select the ip address of machine and on Access tab ..relay option add 127.0.0.1 and server machine address to the list