Help please! When I send an email inside my domain it works fine… but once I send an email outside of the domain, I get the error: “The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay.”

That was a recent request that we received at our Outsourced Technical Support services helpdesk for Web Hosting providers.

Mail servers typically allows all genuine mails. But, if the sender email address appears as fraud, they simply reject mails.

Today, we’ll see what causes this mail error and how our Support Engineers fix it.

What causes “The server rejected one or more recipient addresses” error?

From our experience in managing servers, we usually see this error in Windows based mail servers like Exchange server, MailEnable etc.

The major reasons for this error include:

1. Wrong sender domain

At times, website owners configure the forms that send mails with wrong sender domains. This can be some simple typo errors in the sender domain.

As a result, mail server fails to identify it as a trusted sender and rejects the mail.

2. Sender domain on a different server

Similarly, we see this error when the sender domain has a different IP address pointing to another server.

For security reasons, server administrators configure mail servers to accept mails from its own domains only. Also, it rejects mail from a website that is not pointing to your server. Or, in other words, server is set up as a closed relay.

And in such cases, users will experience problems only when sending mails outside the server.

How to fix “The server rejected one or more recipient addresses” error?

Now that we know the reason for the error, let’s see how to fix it.

Basically, the fix is to make mail server understand that sender is a trusted one. For this, we can easily do 2 things.

1. Change sender domain

Here, we need to change the sender domain in the web form to a valid website on the server. Thus mail server identifies it as trusted and accepts the mail.

2. Allow relay for selected IP address

Alternately, we can add the IP address of the domain as a trusted sender in mail server configuration. The steps differs depending on the exact mail server.

i) In MailEnable

In MailEnable, we can add the IP 127.0.0.1 in the “Allow relay for privileged IP list” from MailEnable Console.  But, this comes with a security risk that any domain hosted on the server can send mails to external domains.

So, our Support Engineers recommends adding the IP of the dedicated server to the privileged IP list. Additionally, we lock down the firewall on the server, so that only authorized IPs can connect to the SMTP port 25.

In a Plesk Onyx server, this privileged IP addresses can be added via Tools & Settings >> Server-Wide Mail Settings option.

ii) In Exchange server

Similarly, in Exchange server, our Support Engineers allow relaying only for specific IP addresses. That way, the server will not function as an open relay for all. Also, we lock down the firewall for the Exchange server, so that only authorized IPs can connect to the SMTP port in the first place.

We modify the list of relay IP addreses and mails flow out successfully.

Conclusion

For security reasons, Mail servers allows traffic only from trusted senders. And, if it suspects a fraud activity, it reject mails with error. Today, we’ve seen the causes for email error “The server rejected one or more recipient addresses” and how our Support Engineers fix them.