ZCS Administrator's Guide 8.0.3
ZCS Administrator's Guide 8.0.3
Open Source Edition

Zimbra Mail Transfer Agent > SMTP Authentication

SMTP Authentication
SMTP authentication allows authorized mail clients from external networks to relay messages through the Zimbra MTA. The user ID and password is sent to the MTA when the SMTP client sends mail so that the MTA can verify if the user is allowed to relay mail.
SMTP Restrictions
You can enable restrictions so that messages are not accepted by Postfix when non-standard or other disapproved behavior is exhibited by an incoming SMTP client. These restrictions provide some protection against spam senders. By default, clients that do not greet with a fully qualified domain name are restricted. DNS based restrictions are also available.
Important: Understand the implications of these restrictions before you implement them. You might have to compromise on these checks to accommodate people outside of your system who have poorly implemented mail systems.
Sending Non Local Mail to a Different Server
You can configure Postfix to send nonlocal mail to a different SMTP server, commonly referred to as a relay or smart host.
A common use case for a relay host is when an ISP requires that all your email be relayed through a designated host, or if you have filtering SMTP proxy servers.
The relay host setting must not be confused with Web mail MTA setting. Relay host is the MTA to which Postfix relays non-local email. Webmail MTA is used by the Zimbra server for composed messages and must be the location of the Postfix server in the Zimbra MTA package.
Configure Relay MTA for external delivery from the administration console, Global Settings>MTA page.
Important: Use caution when setting the relay host to prevent mail loops.
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