In postfix one can change the addresses in the header. No really news for anyone, but usually something you need when you do not expect it. This usually happens when something went wrong and a whole load of messages went out with „.local“ as sender.
Postfix does this with „maps“. If you do not already have one then you generate one. So to mess with this you would use one of two maps.
Here you can do any rewrite, as the word „generic“ may have already had you guessing.