Need help with an LDAP filter

I'm experimenting with using Active Directory as an LDAP source in Turba.
Everything is working great, but I would like to get a search filter working
on it, specifically to only show entries where the email field is not blank
(turba attribute name: "email", AD field name: "mail"). No matter what I
try, e.g.

        'filter' => 'mail <> ""'

Nothing seems to match the filter rule. If I remove the filter, everything
shows up again. Where can I find documenation/examples for the filter()
function?

Thanks!

--Steve


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