User checking for a domain alias

[geefy0]

Hi,

I am having a problem with a domain alias. If I send a test message to validuser@domain.com i get 250 OK. If i send a test to validuser@aliasdomain.com i get 250 OK. However, when i send a test message to invaliduser@domain.com it is rejected, but ACCEPTED by aliasdomain.com. This is causing a problem with user counts on an external spam filter. I need my aliasdomain.com to check if the user is valid for domain.com..

I am having trouble adequately explaining this and I am not sure what terms to Google for, any help would be appreciated. I have searched extensively and can only find how to setup or delete aliases, but nothing on this specific problem. If it would be easier to simply remove the alias that is fine, as long as there is a way to notify anyone sending to it that it has been removed and to use domain.com instead.

Thank you,

--scott