You will need to use the virtusertable for this. In webmin go to sendmail configuration. Then Address Mappings (virtuser). Int the first box "Mail From", you want to click the address button. Then type the full address of the user or virtual user. A virtual user is a name such as webmaster, admin, sales and info. Lets say domain1.com has three users, joe, fred and al. Now you want webmaster's mail to go to joe. Admin's mail to fred and sales and info to al. Here is what the virtusertable would look like:
joe@domain1.com joe
webmaster@domain1.com joe
fred@domain1.com fred
admin@domain1.com fred
al@domain1.com al
sales@domain1.com al
info@domain1.com al
@domain.com error:nouser This server does not allow spam
Notice that the last line has a "catch-all". This line means that if there is no match for bogus-user@domain1.com then return an error message. In this case I have "This server does not allow spam" but you can put whatever you want. Do this for each domain you host. Although you can have duplicate virtual user names, real user names must be unique. I should mention that the destination address can also be an external account i.e. fred@yahoo.com. Once you do this for each of your domains, all you have to do is make sure the domains are listed in Local Domails (Cw). Let me know if you need help setting up relaying for them.