qmail provides the ability to make a copy of each email that flows through the system.
This is done using the QUEUE_EXTRA code. See qmail FAQ #8.2

The qmail tap patch adds additional functionality:
1) Specify which email addresses to tap using a regex style control file. With the
   regex function, you can specify full domains or individual email addresses.

2) Specify which email address to send the emails to.

3) Qmail does not need to be restated to when the taps control file is changed.

The regex match is applied to both the to and from email addresses. So email
sent to or from the addresses will be copied. Matching is case insensitive.
If there are multiple matches, the first match is used.

The queue tap patch adds a new control file:

/var/qmail/control/taps
Contains a regex style list of addresses to tap and the email
address of where you want the copy sent to.

Examples:
a) To tap a whole domain add a line like:
.*@domain.com:joe@example.com


b) To tap an individual email address add a line like:
user@domain.com:other@example.com