Sendmail/postfix pre-queue filter to check envelope sender against To header
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README

Envelope milter
Sendmail/postfix pre-queue filter to check envelope sender against To header

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This milter is useful for MTA's dedicated for outbound mail. The MTA can filter e-mail to make
sure only mail comming from autorized domains is relayed. This is a good messure against spammers
abusing the system, through compromized MUAs, SASL accounts or trusted clients.

This check is done against envelope sender. However the recipient will not see the envelope user,
but the address in the To header. For e-mail comming from a MUA, we can expect the envelope sender
to be the same as the To header, since an MUA should not be relaying e-mail.

This milter defers all mail where the envelope sender is different than the To header.

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Mail with a blank a envelope sender is accepted. A deny rule in the mailserver can block these.

Tested with Postfix 2.5