I suspect %Email only uses a local SMTP, but I won't claim that is 100% true. However, we have our V7 system so BPM uses the local SMTP, and we have the actual target server set up in IIS. So in part it may depend on how your environment is configured.
Presuming your server configuration is in IIS, open the IIS manager and drill down to the Default SMTP Virtual Service | Domains section. Open the properties for the domain Name(s) and the Route domain should have Forward all mail to smart host with the server name entered. Update that value and restart SMTP and you should be all set.