sendmail / multiple domains question

Hello,

I am trying to host multiple domains on my mail server. So, I updated
virtusertable, generics-domains, genericstable. I am able to send and
recive from olduser@1stdomain.com. But I am only able to receive mail
from newuser@2nddomain.com, not send from the account.

 ----- The following addresses had permanent fatal errors ----- 
    (reason: 501 5.1.8 <newuser@2nddomain.com>... Domain of sender address newuser@2nddomain.com does not exist)

Following are the files I modified.
2nddomain is the domain I am trying to add

Virtusertable
----------------------------------------------------------------------------
newuser@1stdomain.com     error:nouser send mail to new@2nddomain.com
newuser@2nddomain.com       newuser
olduser@2nddomain.com      error:nouser user not found
olduser@istdomain.com    olduser

generics-domains(copied local-host-names)
----------------------------------------------------------------------------
1stdomain.com
2nddomain.com

genericstable
----------------------------------------------------------------------------
newuser    newuser@2nddomain.com
olduser    olduser@istdomain.com

Sundar