Control outgoing email of all users?

I'd taxied all mail in an imap account and would teach "that man with the big ears" to shift the letters to the client in two different folders
Simple program to periodically deflate from a folder letters and forwards outbound. Of course, clean the folder a and the folder with rejected just clean.
A bit strange Received headers will work, but it could be worse. They can be in the same program to clean.

always_bcc_to in the case of Postfix.

Is a very complex task, considering that a piece of mail sent through the web interface of Gmail and other.
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Would recommend you to intercept means the firewall all connections on port 25, and turn them into your MTA (e.g. Exim), which will allow you to do with the post almost everything you want. Note, however, the client can say STARTTLS and the certificate will not agree with that which he wishes to obtain, and in the headers of sent emails, you will see that they have passed through your server.
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Transparent to the user method includes the analysis of intercepted traffic, or man-in-the-middle: doing anything like antivirus software that checks email. Here is an example similar softiny for UNIX-systems: software.klolik.org/smtp-gated/
Maybe you'll find something else, and thus more appropriate for your purposes
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Do not exclude also that there is a ready commercial software for such cases, and doing their thing quietly, and perhaps even under Windows — domestic bezopasnym this crap often goes in the head

Easier to put the keypad. spy and send data to the server, which is then read.

TK will clarify: decision is necessary for the accounts to our Myl'nikov domain_name.com.
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Ie and domain and hosting and have the MX host, I want users to configure accounts, for example, The Bat or Thunderbird species user@domain_name.com. web we have closed.

Then the problem is reduced by a few lines of config MTA (a one-line version of the proposed first).
thank you, but could you specify more precisely what the options mean.
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the single-line version does not correspond to TK, because they accumulate in the back box mail will have to manually forwarding that changes the form of a letter.