Zimbra Relay Access Denied May 2026

By default, Zimbra’s Postfix (the MTA underneath) is configured as a closed relay. This prevents spammers from abusing your server to send thousands of emails to Gmail or Yahoo. When you see "Relay Access Denied," Zimbra is saying: "I don’t know this sender, and I’m not responsible for the destination domain—so I’m refusing this message."

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Start with authentication (port 587). If that doesn’t work, check your mynetworks . Nine times out of ten, that resolves the issue. By default, Zimbra’s Postfix (the MTA underneath) is

This most often happens in three specific scenarios: Zimbra’s default security stance is: Authenticate first, then relay. If a device or script tries to send mail through your server on port 25 (the standard SMTP port) without a username and password, Zimbra will reject it. Subscribe to our newsletter for more Zimbra and