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<li> <p> The queue manager (the qmgr(8) server process in the
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figure) is the heart of Postfix mail delivery. It contacts the
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smtp(8), lmtp(8), local(8), virtual(8), pipe(8), or error(8) delivery
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agents, and sends a delivery request for one or more recipient
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addresses. The error(8) delivery agent is special: it always declares
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mail as undeliverable. It is not shown in the figure above. </p>
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smtp(8), lmtp(8), local(8), virtual(8), pipe(8), discard(8) or
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error(8) delivery agents, and sends a delivery request for one
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or more recipient addresses. The discard(8) and error(8) delivery
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agents are special: they discard or bounce all mail, and are not
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shown in the figure above. </p>
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<p> The queue manager maintains a small active queue with the
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messages that it has opened for delivery. The active queue acts as
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<p> The previous sections gave an overview of how Postfix server
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processes send and receive mail. These server processes rely on
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other server processes that do things behind the scenes. Where
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practical, each service will be visualized in its own context. As
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other server processes that do things behind the scenes. The text
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below attempts to visualize each service in its own context. As
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before, names followed by a number are Postfix commands or server
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programs, while unnumbered names inside shaded areas represent
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Postfix queues. </p>
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<li> <p> The anvil(8) server implements client connection and rate
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<li> <p> The anvil(8) server implements client connection and
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limiting for all smtpd(8) servers. The TUNING_README document
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provides guidance for dealing with mis-behaving SMTP clients. The
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anvil(8) service is not included with Postfix version 2.1 or earlier.
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anvil(8) service is available in Postfix version 2.2 and later.
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and reduces the number of open lookup tables by sharing one open
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table among multiple processes. </p>
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<li> <p> The scache(8) server maintains the connection cache for
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the Postfix smtp(8) client. When connection caching is enabled for
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selected destinations, the smtp(8) client does not disconnect
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immediately after a mail transaction, but gives the connection to
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the connection cache server which keeps the connection open for a
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limited amount of time. The smtp(8) client continues with some
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other mail delivery request. Meanwhile, any smtp(8) process can
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ask the scache(8) server for that cached connection and reuse it
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for mail delivery. As a safety measure, Postfix limits the number
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of times that a connection may be reused. </p>
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<p> When delivering mail to a destination with multiple mail servers,
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connection caching can help to skip over a non-responding server,
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and thus dramatically speed up delivery. </p>
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<tr> <td align="center" bgcolor="#f0f0ff"> <br> smtp(8) <br>
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</td> <td> <tt> <-> </tt> </td> <td align="center"
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bgcolor="#f0f0ff"> <br> scache(8) <br> </td> <td> <tt>
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<-> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br>
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smtp(8) <br> </td>
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<li> <p> The showq(8) servers list the Postfix queue status. This
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is the queue listing service that does the work for the mailq(1)
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and postqueue(1) commands. </p>
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standard input, output and error streams. You can find examples of
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its use in the SMTPD_POLICY_README document. </p>
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<li> <p> The tlsmgr(8) server runs when TLS (Transport Layer
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Security, formerly known as SSL) is turned on in the Postfix smtp(8)
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client or smtpd(8) server. This process has two duties: </p>
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<li> <p> Maintain the pseudo-random number generator (PRNG) that
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is used to seed the TLS engines in Postfix smtp(8) client or smtpd(8)
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server processes. The state of this PRNG is periodically saved to
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a file, and is read when tlsmgr(8) starts up. </p>
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<li> <p> Maintain the optional Postfix smtp(8) client or smtpd(8)
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server caches with TLS session keys. Saved keys can improve
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performance by reducing the amount of computation at the start of
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<p> TLS support is available in Postfix version 2.2 and later.
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Information about the Postfix TLS implementation is in the TLS_README
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<tr> <td>Network<tt>-> </tt> </td> <td align="center"
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bgcolor="#f0f0ff"> <br> smtpd(8) <br> </td> <td colspan="2">
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<tt> <---seed---<br><br><-session-> </tt> </td> <td
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align="center" bgcolor="#f0f0ff"> <br> tlsmgr(8) <br> </td>
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<td colspan="3"> <tt> ---seed---><br> <br><-session->
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</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> smtp(8) <br>
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</td> <td> <tt> -></tt>Network </td> </tr>
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<tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td>
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</td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table>
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</td> <td align="center"> |<br> |</td> <td align="left"> <table>
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<tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td>
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</tr> </table> </td> <td colspan="3"> </td> </tr>
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<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
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smtpd<br> session<br> cache </td> <td> </td> <td align="center"
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bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td
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align="center" bgcolor="#f0f0ff"> smtp<br> session<br> cache </td>
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<td colspan="2"> </td> </tr>
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<li> <p> The verify(8) server verifies that a sender or recipient
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address is deliverable before the smtpd(8) server accepts it. The
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verify(8) server injects probe messages into the Postfix queue and
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proto/OVERVIEW.html
