What Exactly Does The Error Mean?
Postfix is by default equipped with a number of controls and safeguards to prevent malicious
This error is returned when Postfix user attempts to send an email and it’s rejected by the target domain. It usually means the user isn’t properly authorized – either they’re connecting from a non-trusted domain or they aren’t targeting a trusted destination.
What Could Be Causing It?
Misconfiguration, more often than not, though occasionally DNS issues could also be the cause. Postfix’s SMTP access restrictions, though designed to cut down on spam, can often prevent your users from sending emails if they’re not properly set up. There’s also the chance that the problem isn’t on your end, but on the recipient’s.
What’s The Solution?
There are a few steps you’ll want to take in order to fix this error, depending on what’s causing it(Source):
- First, check to make sure your Postfix is properly configured. Read the document linked at the end of this section for more details on how to do so.
- Next, verify that you’re using the same DNS as your mail server. If you fail to do this, you could end up receiving inaccurate results from your tests.
- Figure out what mail server your problem domain is using. There are a number of different ways you can do this, such as MX Lookup.
- Start a telnet prompt, with the problem domain as the target. Identify yourself as your mail server, then identify a test account on the problem domain as the recipient. If you get a 510 Bad User error, that means the problem is with your server.
- If you receive a 550 Relaying Denied error, change your DNS server to an external one – and make sure it’s a known ‘good’ server. If the test is successful, then you’ve found the problem.
- If the test fails, check to see if you’re on any Realtime Blackhole Lists, make sure you’re running an open relay, and contact your host.
It is highly recommended that you read the Postfix SMTP relay and access control document. It’s vital that you understand the access controls and safeguards available to you – both for your protection and to ensure you properly configure things in the future.