Please help! When I send mails to two of my friends, one of them never receives it and I get a mail bounce.

That was a recent support request that we received at our Outsourced Tech Support Services, where we resolve support queries for web hosting providers.

The hosting user kept on getting the error “550 permanent failure for one or more recipients” from one of her contacts.

Why did this happen? Let’s dive into the details:

What causes “550 permanent failure blocked” email error?

The exact bounce mail received by the customer was :

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

melissa.b@xyz.com
host srv.mailer.com [168.xx.xx.105]
SMTP error from remote mail server after end of data:
550 permanent failure for one or more recipients (melissa.b@xyz.com:blocked)

From the bounce message, it was clear that the recipient mail server blocked mail to melissa.b@xyz.com.

To correct this, we need to first find the cause for the error.

So, our Support Engineers debugged the possible reasons for recipient server to block emails with the error “550 permanent failure” which include :

1. Sending server IP blacklisted.

Spam monitoring companies like Barracuda, Spamhaus etc. maintain a list of servers that frequently send spam mails.

To fight against spam, most mail servers block mails from such blacklisted servers.

Here in our case, we confirmed that our mail server was not listed in any blacklists and it had a very good web reputation.

So obviously that was not the reason for mail failure.

Another possible cause for the error was that somehow recipient server assumed sender as fraud.

Let us check how this can happen.

2. Bad SPF record for domain

Many email servers use a security feature called SPF (Sender Policy Framework) to ensure authenticity of email sender.

This SPF record decides the servers that can send mail on behalf of the domain.

When mails originate from servers that are not mentioned in SPF record of domain, mail servers assume them as fraud mails and reject them.

Here, in this case, the domain xyz.com was using a custom Spam filter service from the Web Host. But, the IP address for Spam filter was missing in the SPF record and server rejected the mail.

We corrected the SPF record for xyz.com as :

xyz.com. 600 IN TXT “v=spf1 a mx a:spamfilter.srv.com ~all”

We added the entry spamfilter.srv.com in the SPF record for xyz.com.

After that, we sent a test mail to melissa.b@xyz.com. This time the spf check by recipient server passed and mail was successfully delivered.

Conclusion

“550 permanently failure blocked” email error mainly happens due to the restrictions at recipient mail server. Today we’ve seen how our Support Engineers corrected the sender domain settings and fixed the error.