I was wondering if there is a way for the following setup requirement:

Postfix handles mail from any sender, but when the specifice sender like 'sender@fromdomain.tld' sends mail, postfix should not send the mail but let the mail be deliverd by smtpserver.forrelay.tld.

Hopefully the question is clear. Is this possible?

(In this case setting the mail-client to use a different SMTP server for the sender isn't possible)