Allowing only certain senders to send mail to an address

[Thomas K�nig]

How I have to configure qmail / ezmlm, so that mails to a specified 
account e.g. [EMAIL PROTECTED], only will be accepted, if there
comes from a certain ip-address.

The Background is:
Users are only can subscribe and unsubscribe, via webinterface,
and not directly via mail.
The request for confirmation and the answer from the users are be 
delivered directly and not restricted.

How can I do that?


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thomas koenig