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How can I configure exim4 (or another MTA) to avoid sending duplicate emails?

For my purposes a duplicate email can be defined as: same recipient and subject (or same recipient, subject and body)

I want to ensure only a maximum of 1 of these is sent per 24 hours. Any others should not even be queued.

Is this possible with exim4? If not, what's the best way of going about it?

This is for a safeguard against application bugs.

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With postfix or sendmail I would install MIMEDefang and then modify filter_recipient() so that I would keep the (sender,recipient,subject,hash of body) tupple. Either the tupple is already in a database and thus the mail is discarded, or it is not and thus the mail is allowed to pass through with the tupple describing it logged in the database.

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