355
recipient will be accepted immediately. In contrast an email will be delayed if is unknown
or it timed out.
It depends on the configuration of the sending relay server when
and how often it will retry delivery. SX-GATE has no influence
on this. In most cases the retransmission will take place in less
then an hour. However longer delays are possible. It is even
possible that some servers will not retry at all. However in this
case the sender is usually notified. There is also a builtin whitelist
of important servers which are known not to retransmit.
To avoid unwanted delays, greylisting can be disabled for certain senders or recipients.
Greylisting will never be effective for
•
connections from a local source IP address (see "Local IP addresses" in
menu "Modules > Mail Server > SMTP settings" on tab "Relay control")
•
connections from IP addresses included in the builtin whitelist of servers
known to perform no retransmissions
•
emails retrieved from POP servers
•
authenticated connections (SMTP auth)
Working method
This switch will enable greylisting. As described above, greylisting is useful if incoming
mails are directly delivered with SMTP. Hence for at least one local domain the Internet
DNS mail exchanger (MX) must point to SX-GATE's external IP address.
Any backup MX entries need to be removed from DNS. This
does not apply to backup servers which do not accept emails
while SX-GATE is alive. Backup servers which apply greylisting
themselves are acceptable, too. Adapt the greylist timings to
those of the backup.
A setup where incoming emails are forwarded to SX-GATE
through an upstream mail relay is not suitable for greylisting.
However switching to direct delivery shouldn't be a problem in
this case.
check DNS blacklist
This is the most tolerant greylisting mode. Hence it can be enabled without
problems on most SX-GATEs, provided the prerequisites for greylisting are met.
Once an incoming connection has been established, SX-GATE looks up the
