Zimbra Relay Access Denied May 2026

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.

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. zimbra relay access denied

Found this helpful? Subscribe to our newsletter for more Zimbra and open-source mail server tips. It usually appears without warning

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: This error is actually Zimbra’s security system doing

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.