Outbound spam filter and forwarding messages

[Karandras]

Hey all, the company we use for our outbound spam filter for our www clients has informed me the only way around my problem is to re-write the sender address in postfix. Here is the problem and please tell me if that's the only way around...if it is then I might need to get pointed in the right direction:

Client fills out a form on a website which emails a confirmation from a non-valid email address:

==Some starter info==
---We host domain.com and email for [email protected].
---We do not host otherdomain.com.
---We forward all outbound email through outgoing.emailscanner.com.

==Here is the message flow==
-Client fills out form
-Website emails form to client's email address [email protected] from [email protected]
-emailscanner.com Accepts email from the Website for [email protected]
-emailscanner.com scans and verifies not spam. It sends to our plesk server.
-Our plesk server accepts the email from emailscanner.com for [email protected]
-Plesk sees there is a forward on [email protected] -> [email protected]
-Plesk forwards email to [email protected] from [email protected] through the smarthost outgoing.emailscanner.com
-outgoing.emailscanner.com accepts email from Plesk to [email protected]
-outgoing.emailscanner.com scans email from [email protected].
-outgoing.emailscanner.com attempts to verify the email address, it doesn't exist.
-outgoing.emailscanner.com sends message back to plesk indicating not a valid email address
-plesk accepts the message and attempts to deliver to [email protected], not an email address on this server.
-plesk puts into the deferred queue with error (host outgoing.emailscanner.com[1.1.1.1] said: 451 unable to verify sender (in reply to MAIL FROM command))

Any help here would be much appreciated.
Thanks!