Add the external domain to the list of allowed "From" addresses:
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.
Add the device’s IP address to Zimbra’s “mynetworks” setting. This tells Zimbra, "Trust anything coming from this IP."
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 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."
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.
