I might be wrong (and if I am hopefully someone can correct me)- but my understanding is, that once the email is sent, any moodle filters (e.g. filtercodes, Generico or any other filter) won't work inside the email, as there is no longer the direct connection back to the Moodle database that effectively swaps the filter syntax out, and replaces it with the desired code.
Some of the emails that come out of Moodle, have wildcards as part of them, e.g. assignment submission emails - and with these the variable text is injected into the email before it is sent. But the forum email's do not have this functionality.