MTA stack service is used by Exchange 5.5. In Exchange 2003 its been taken over by SMTP service. So if you do not have Exchange 5.5 you can turn of the MTA stack service. If this is the case then the OWA issue is totaly different and not realted to MTA stack service and you have to check with IIS setting to troubleshoot OWA issue. Manoj Enterprise Messaging Support