[Exim] another exim problem with smarthosts and $header_fro…

hi *

my old problem with multiple smarthosts seems to be solved - thanks to John
Horne - but now, i detected a new problem: i have e-mail-adresses like this:
person@???_b.org (and person@???_b.org) - how can i
solve this with exim?

i tried
${if match{$header_from:}{\\@third\\.*\\$}{mail.account_x.org}

but this didn't work, he didn't taked mail.account_x.org :(

frank