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Postfix Documentation
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Configuring a domain for ETRN service only

While an "ETRN" customer is off-line, Postfix will make spontaneous attempts to deliver mail to it. These attempts are separated in time by increasing time intervals, ranging from $ minimal_backoff_time to $ maximal_backoff_time, and should not be a problem unless a lot of mail is queued.

To prevent Postfix from making spontaneous delivery attempts you can configure Postfix to always defer mail for the "ETRN" customer. Mail is delivered only after the ETRN command or with "sendmail -q", with "sendmail -qRdomain", or with "postqueue -sdomain"(Postfix version 1.1 and later only),

In the example below we configure an "etrn-only" delivery transport which is simply a duplicate of the "smtp" and "relay" mail delivery transports. The only difference is that mail destined for this delivery transport is deferred as soon as it arrives.

 1 /etc/postfix/master.cf:
 2   # =============================================================
 3   # service type  private unpriv  chroot  wakeup  maxproc command
 4   #               (yes)   (yes)   (yes)   (never) (100)
 5   # =============================================================
 6   smtp      unix  -       -       n       -       -       smtp
 7   relay     unix  -       -       n       -       -       smtp
 8   etrn-only unix  -       -       n       -       -       smtp
 9 
10 /etc/postfix/main.cf:
11   
relay_domains = customer.tld ...other domains...
12   
defer_transports = etrn-only
13   
transport_maps = hash:/etc/postfix/transport
14
15 /etc/postfix/transport:
16   customer.tld     etrn-only:[mailhost.customer.tld]

Translation:

  • Line 8: The "etrn-only" mail delivery service is a copy of the "smtp" and "relay" service.

  • Line 11: Don't forget to authorize relaying for this customer, either via relay_domains or with the permit_mx_backup feature.

  • Line 12: The "etrn-only" mail delivery service is configured so that spontaneous mail delivery is disabled.

  • Lines 13-16: Mail for the customer is given to the "etrn-only" mail delivery service.

  • Line 16: The "[mailhost.customer.tld]" turns off MX record lookups; you must specify this if your Postfix server is the primary MX host for the customer's domain.

Postfix Documentation
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