Duplicate Delivery Suppression
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Duplicate delivery suppression is a feature of the Cyrus IMAP server
that allows multiple copies of an identical messages delivered to one
user to be suppressed, so that the user only receives one copy. This
can be convenient if, say, one user is on multiple mailing lists that
commonly receive the same message.

The way that a message is determined to be a duplicate is a lookup is
done in the duplicate delivery database for a message-id/mailbox pair.
If a match is found, the message is suppressed. If a match is not
found, the pair is added to the database and the message is delivered.

Duplicate Delivery Suppression can also affect sieve redirects. In
this case, suppression is done on a message-id/redirect-target basis.

Duplicate Delivery Suppression can also affect vacation messages. In
this case, suppression is done based on a hash of the sender’s address
and the vacation message.
