Authhost, noauthhost, defaulthost, nodefaulthost, remotehost, noremotehost Channel Options

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Host name to use when correcting incomplete addresses (auththost, noauthhost,defaulthost, nodefaulthost, remotehost, noremotehost)

The MTA often receives addresses consisting of a bare local-part and no "@domain" from misconfigured or incompliant mailers and SMTP clients. (Note that the standards do in fact require acceptance of one special address - "postmaster" in SMTP, but do not state how such an address is to be represented in a message header.) This happens often enough that simply disallowing such addresses is rarely an acceptable strategy.

The MTA, showing at least some respect for standards, must attempt to make such addresses legal before passing the message along. The MTA does this by appending a domain name to the address (e.g., appends "@acme.com" to "mrochek", producing "mrochek@acme.com").

The set of options described here control how this domain name is selected. In this process the MTA makes a distinction between envelope To (RCPT TO) addresses versus addresses appearing in all other contexts (header, MAIL FROM), as well as distinguishing SUBMIT from SMTP.

In the following table showing what domain is used when various options or protocols are used, o.org is the domain attached to the local channel, a.com is the domain associated with the current authenticated user's primary email address, l.com is the first argument of the defaulthost option setting on the current source channel, r.com is the second argument of the defaulthost option setting on the current source channel, and r.edu is the domain associated with the remote SUBMIT/SMTP client.

Options controlling missing domain fixups
Option Protocol Header/MAIL FROM RCPT TO
authhost (and authenticated) SUBMIT/SMTP @a.com @a.com
defaulthost SUBMIT/SMTP @o.org @o.org
defaulthost l.com SUBMIT @l.com @l.com
defaulthost l.com SMTP @l.com @o.org
defaulthost l.com r.com SUBMIT @l.com @l.com
defaulthost l.com r.com SMTP @l.com @r.com
nodefaulthost (MS 8.0.2.1 or later) SUBMIT/SMTP <no fixup> <no fixup>
remotehost SUBMIT @r.edu @r.edu
remotethost SMTP @r.edu @o.org
<no options> SUBMIT/SMTP @o.org @o.org

Note: The option in the preceding table are shown in precedence order, that is, a given option setting will override option settings further down the table.

The usual configuration is to have both arguments to defaulthost set to the defaultdomain on the defaults channel, overriding any use of the local channel host. It may be appropriate in multitenant configurations to set authhost on all channels that are marked with the submit channel option

Use of the remotehost channel option may be considered in rare cases where remote SMTP client exist in another administrative domain which partial addresses to refer to their own users.

Note that rewrite rules can make use of whatever default has been selected via the the $G and $nG substitutions.

Note that the switchchannel, saslswitchchannel, tlsswitchchannel and various other options can be used to associated incoming SMTP connections with a particular channel, and thus control what set of options are used.


See also: