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SMTP error

How to fix email error “User unknown in local recipient table”

In an ideal world, emails will work flawlessly and you never have to pull your hair out.

Unfortunately, we live in an imperfect world, where mails bounce with weird errors such as “User unknown in local recipient table“.

What is this table? How do I fix it? Out of all people in the world, why should I get this error?

Here we’ll see the answer for all these questions.

At Bobcares, we help website owners to fix these email bounce errors as part of our Outsourced Tech Support services for Web Hosts.

We’ve seen broadly 3 causes for the error “User unknown in local recipient table“.

Today we’ll discuss each of these causes and how we fix them.


5 causes for “SMTP error from remote mail server after RCPT TO” error, and the fixes

Email errors can be really bothersome – they can hit you hard and unexpected, when you attempt to send an urgent mail.

As part of our Outsourced Hosting Support services for web hosting providers, we resolve email errors faced by website owners. (more…)

Error “Failed to authenticate on SMTP server with username” – Causes & fixes

Almost every PHP application relies on mail functions to send contact requests, newsletters, and more.

The most secure way to send these mails is to use SMTP authentication – that is, the PHP app uses a username and password to transfer mails to an SMTP server.

But this authentication can sometimes fail with the error:

Failed to authenticate on SMTP server with username "" using XX possible authenticators


Failed to connect to mailserver at – 5 causes & fixes for this PHP error

Here at, we provide Outsourced Tech Support to web hosting providers. As part of our service, we resolve website errors posted by hosting users.

A common error we’ve seen in PHP applications is:

Warning: mail() [function.mail]: Failed to connect to mailserver at "localhost" port 25, verify your "SMTP" and "smtp_port" setting in php.ini