[Exim] Exim Filter Problems

I'm trying to use the deliver command with a username that contains an '@'
sign.

I've tried:
$local_part/folder1.subfolder2
user@suffix/folder1.subfolder2
user\@suffix/folder1.subfolder2
user\\\@suffix/folder1.subfolder2

All seem to want to be converted into a real mail address:
  user@suffix/folder1.folder2
    (ultimately generated from user@???)
    mail domain "suffix/folder1.folder2" is syntactically invalid

Any ideas how to correct this?

Thanks,

Matt