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.
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
relay_domains = customer.tld ...other domains...
defer_transports = etrn-only
transport_maps = hash:/etc/postfix/transport
16 customer.tld etrn-only:[mailhost.customer.tld]
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
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.