What causes “Sender verify failed” SMTP mail error, and how to resolve it in ISPConfig Postfix servers

SMTP error from remote mail server after RCPT TO:<user2@domain2.com>:
host mx.server.com [xxx.xxx.xxx.xxx]: 550-Verification failed for <user1@domain1.com>
550-The mail server could not deliver mail to user1@domain1.com. The account ordomain may not exist, they may be blacklisted, or missing the proper dns entries.
550 Sender verify failed

If you are a ISPConfig web host, you would recognize this as one of the common errors reported by your customers.

Sender verify failed can be caused by issues ranging from improper file permissions to RFC non-compliance. Engineers at Bobcares help desk support services collect detailed information regarding the error through log analysis and engaging the customer. The following are the few of the common causes for this error.


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Email account doesnt exist or cannot be detected

When a user tries to send a mail through the server, the sending mail address needs to be present and active in the server. In recently migrated domains, it is possible that the email accounts might not be setup in the server. The customer would try to connect using the old email username, but the server would return an error as it cannot find a matching account. Such situations are quickly fixed by creating the corresponding email accounts.

Duplicate sender account in recipient server

Sometimes it is possible that a user is unable to send mails to domains with his old service provider, or to another server within our network. For example, domainONE.com and domainTWO.com used to be with the same provider. domainONE.com migrated in to our servers, but is now unable to send mails to domainTWO.com. This happens because the name servers of domainTWO.com would still have entries of domainONE.com.

In such situations, we contact the recipient server’s administrators and get the phantom account deleted from their servers. In case, the recipient server is in our network, we delete the duplicate account from that server.

If web masters report that their contacts are unable to mail them because of Sender verify failed, we check the Postfix logs to see if the failure was due to Address verification failed. Some servers use tar-pitting as an anti-spam defense, and if our server timed-out trying to establish a verification connection, the email would not be accepted. To resolve this, we reconfigure Postfix to whitelist the recipient server.

Sender verify failed could be caused by other circumstances as well. If you do not see a solution for your particular issue listed above, just leave a comment with the details of the error below, and we would be happy to answer.

To check out other common support issues resolved by us, take a look at our help desk support plan.

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Bobcares is a server management company that helps businesses deliver uninterrupted and secure online services. Our engineers manage close to 51,500 servers that include virtualized servers, cloud infrastructure, physical server clusters, and more.