<header>Canonical Mapping</header>
The optional canonical file specifies an address mapping for local and
non-local addresses. The mapping is used by the cleanup(8) daemon. The address
mapping is recursive.
<p>
The file serves as input to the postmap(1) command. The result, an indexed file
in dbm or db format, is used for fast searching by the mail system.
<p>
The canonical mapping affects both message header addresses (i.e. addresses
that appear inside messages) and message envelope addresses (for example, the
addresses that are used in SMTP protocol commands). Think Sendmail rule set
<tt>S3</tt>, if you like.
<p>
Typically, one would use the canonical table to replace login names by
<tt>Firstname.Lastname</tt>, or to clean up addresses produced by legacy mail
systems.
<p>
The canonical mapping is not to be confused with virtual domain support. Use
the <a href=virtual>virtual domain</a> map for that purpose.
<p>
The canonical mapping is not to be confused with local aliasing. Use the
<a href=aliases>mail aliases</a> map for that purpose.
<p>
The format of the canonical mappings is as follows, mappings being tried in the
order as listed:
<ul>
<li><b><tt>user@domain</tt></b> mapped to... <b><tt>address</tt></b>:<br><tt>user@domain</tt> is
replaced by <tt>address</tt> This form has the highest precedence. This form
useful to clean up addresses produced by legacy mail systems. It can also be
used to produce <tt>Firstname.Lastname</tt> style addresses, but see below for a
simpler solution.
<li><b><tt>user</tt></b> mapped to...<b><tt>address</tt></b>:<br><tt>user@site</tt> is replaced
by address when site is equal to <tt>$myorigin</tt>, when site is listed in
<tt>$mydestination</tt>, or when it is listed in <tt>$inet_interfaces</tt>.
This form is useful for replacing login names by <tt>Firstname.Lastname</tt>.
<li><b><tt>@domain</tt></b> mapped to...<b><tt>address</tt></b>:<br>Every address in domain is
replaced by address. This form has the lowest precedence.
</ul>
In all the above forms, when address has the form <tt>@otherdomain</tt>, the result is
the same user in otherdomain.
<p>
ADDRESS EXTENSION: When table lookup fails, and the address localpart contains
the optional recipient delimiter (e.g., <tt>user+foo@domain</tt>), the search
is repeated for the unextended address (e.g. <tt>user@domain</tt>), and the
unmatched extension is propagated to the result of table lookup. The matching
order is: <tt>user+foo@domain</tt>, <tt>user@domain</tt>, <tt>user+foo</tt>,
<tt>user</tt>, and <tt>@domain</tt>.
<hr>
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