how to have mail go out on different interfaces???

[Jeremy Hansen]

I have three domains.  If I send mail from domain1, I would like it to go
out interface 1, if I send mail from domain2, I want it to go out
interface2, if I send mail from domain3, I want it to go out interface3.
Is this possible?  Does this have to do wiht bindroutes? 

Thanks
-jeremy