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Enable this option only if all emails with local sender domains
are always sent to the Internet via local systems (SX-GATE or
internal mailserver).
Verify internal addresses in advance
When this option is enabled, SX-GATE will contact the internal mail server for every
email it receives to verify if it is willing to accept a message for the given recipients.
This is checked before the actual message body is transmitted to SX-GATE, so mails
to non-existent recipients will be rejected before wasting bandwidth.
This feature applies to all recipient domains SX-GATE forwards
to an internal mail server.
Address verification also applies to mails SX-GATE retrieves
from a POP or IMAP server. If the internal mail server refuses
delivery, the mail is usually silently discarded.
using SMTP
This is the most simple approach which works with almost all mail servers. For
each inbound mail SX-GATE opens an SMTP connection to the internal mail
server and The received sender and recipient addresses are forwarded to it.
Depending on the replies of the internal mail server, the sender is then either
allowed to continue with the transmission of the email or the mail is rejected.
With this method you can even make use of any capability
your internal mail server offers to reject sender addresses.
Make sure that the internal mail server immediately rejects
unknown recipient addresses. In Microsoft Exchange you
might have to enable recipient filtering. Since Exchange
2013 this requires an additional HubTransport connector
in Exchange, scoped for SX-GATE's IP and granting
anonymous access.
