Adding postmaster address for multiple domains

Hello all,

Our company has many different domains and it appears none of them have a postmaster@domain.com  address. I was easily able to create the postmaster account for our main domain using Get-TransportConfig | Format-List ExternalPostmasterAddress and Set-TransportConfig -ExternalPostmasterAddress postmaster@domain.com 

What I am asking is how do I go about configuring the postmaster for our other domains? It can't be as simple as doing Set-TransportConfig -ExternalPostmasterAddress postmaster@domain2 .com? I feel like that would just set the main domain postmaster address to postmaster@domain2.com.

Thanks for any help you can give.