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.
It usually appears without warning. One minute, a user or an application is sending mail fine; the next, emails are bouncing back. Don’t panic. This error is actually Zimbra’s security system doing its job—it just needs a little adjustment.
If you manage a Zimbra Collaboration Suite (ZCS) environment, you’ve likely seen the dreaded "554 5.7.1 <[email protected]>: Relay access denied" error in your mail logs. zimbra relay access denied
Change the sending device to use port 587 (Submission) instead of port 25, and enable SMTP Authentication . Most modern email clients (Outlook, Thunderbird, Apple Mail) support this natively.
zmprov modifyAccount [email protected] +zimbraAllowFromAddress [email protected] zmprov fc account [email protected] This is a classic "broken copier" or "buggy CRM" problem. Printers, scanners, and legacy applications often hard-code an IP address and try to send mail without logging in. This most often happens in three specific scenarios:
Add the device’s IP address to Zimbra’s “mynetworks” setting. This tells Zimbra, "Trust anything coming from this IP."
Add the external domain to the list of allowed "From" addresses: One minute, a user or an application is
zmprov modifyServer `zmhostname` zimbraMtaMyNetworks '127.0.0.0/8 10.0.0.0/24 YOUR_DEVICE_IP/32' zmcontrol restart mta Only do this for internal, static IPs. Never add public IP ranges here. How to Diagnose the Problem in 30 Seconds Still stuck? Check the mail logs. SSH into your Zimbra server and run:
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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."
In this post, we’ll break down why this happens and the three most common ways to fix it. An SMTP relay is when a mail server accepts a message and delivers it to a domain that is not its own.