There are articles on communities for this already - the latest is here Notification configurition in BSA
The BMC docs article is here https://docs.bmc.com/docs/display/public/bsa83/Configuring+a+mail+server
A simple summary is to set the values for all application server instances with blasadmin and restart the application servers
blasadmin -a set emailconfig smtpserver "mailservername:port" - port is optional not required if running on 25
blasadmin -a set emailconfig fromaddress "email@example.com"