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source IP in several realtime databases of known SPAM senders and dynamic IP
addresses. Only when this lookup succeeds, greylisting will be imposed.
Exchange2k/catch-all mode
This option has been included for two special cases. When selected, the sender
of each outbound email will be whitelisted as recipient for inbound mail. Each
succeeding outbound mail will renew the clearance, otherwise it will expire after
the period of time configured in "Timeout after last use". The run-in phase of this
configuration is very short, as every local user has to send but a single mail.
Important local email addresses which are rarely or even
never used as the sender address of outbound mail should
be added manually to the "Recipient whitelist".
Do not select this option if your setup does not correspond
to one of the following. Otherwise greylisting will be of little
avail.
Select this option if SX-GATE forwards emails to an internal Microsoft Exchange
server release 2000 or older. These servers are not able to reject emails to
unknown local recipients beforehand. Instead every mail is accepted and, if the
recipient does not exist, a non-delivery response is returned to the sender. So
the number of mails to process will almost double, which may have a significant
impact on the whole infrastructure. Even worse, in case of a SPAM mail, the non-
delivery report is often sent to a faked or non-existent address, which leads to
even more annoyance. SX-GATE's particular greylisting mode can help to reduce
the amount of non-delivery reports, as every mail which has been addressed to
an unknown local recipient has to overcome greylisting.
From Exchange 2003 on it is able to reject emails
to unknown local recipients immediately and SX-GATE
can check this before accepting an inbound mail. For
more information please check the option "Verify internal
addresses in advance" in menu "Modules > Mail Server >
SMTP settings" on tab "Receiving filters".
The second use case are so called "catch-all" email domains. Here, when no
local user is associated with the actual recipient address, the mail is delivered
to a dedicated mailbox or group account. The majority of emails here is most
likely SPAM to random recipient addresses. The best solution to this problem is
probably to get rid of the catch-all behaviour. If this is an option and SX-GATE
forwards mails to an internal mail server, please switch to menu "Modules > Mail
Server > SMTP settings", tab "Receiving filters", enable "Verify internal addresses
in advance" and disable the catch-all feature in the configuration of the internal
