The Relay MTA global setting is used to instruct the MTA to send all non-local mail to a different SMTP server. Common use cases for a relay host is when an ISP requires that all email messages be relayed through a designated host or if you have some filtering SMTP proxy server.
This relay host settings should not be confused with the 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.
Important: Use caution when setting a relay host to prevent mail loops.
Procedure
1. Go to the Configure>Global Settings>MTA page.
2. In the Network section, in the Relay MTA for external delivery text box, add the host name and port.
3. Click Save.
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