Sendmail - relay unknown user mail to ISP

[nocolour]

Use sendmail to forward or relay unknown user mail to ISP

How to relay unkown user mail to ISP (hosting server) ???
what command I have to add?

If I use :
define(`SMART_HOST',`hosting.mydomain.com')
FEATURE(relay_entire_domain)

All mail will send to "hosting.mydomain.com" , increase my internet trafic ..~!!!!

I just want send "unknown@mydomain.com" to hosting server

help!!! Thanks.....