[exim] exim filter, password check like htpasswd

in a filter script I check the from address for containing a certain
domain. All other senders shall not be allowed to use this mail address.
This works fine.
As extension of this I would like to give the sender from 'external' the
possibility to authenticate with a password to override the limitation
of the filter.
Has anybody a idea for a solution?

My filterscript:
#Exim filter
if $h_from: does not contain "mydomain" then
save /home/...../Spam/
seen finish
endif


Many thanks
Rainer sigl

-- 
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Rainer Sigl,                      | email: sigl@???
MPI f. extraterrestrische Physik, | phone: +49 (89) 30000-3557
Postfach 1312,                    | fax:   +49 (89) 30000-3569
D-85741 Garching, Germany         | web:   http://www.mpe.mpg.de/~sigl/