SMTP Error (535): Authentication failed – How we Fixed it

Table of Contents

Everyone expect mails to work flawlessly, but that doesn’t always work.

Occasionally, mail servers may reject emails with some strange errors, such as “SMTP Error (535): Authentication failed

At 1 onlyhost, we often get requests from customers to fix SMTP errors as part of our Technical Support Services.

Today, we’ll see how our Support Engineers help our customers to resolve such tough email errors.



What is SMTP Authentication?

Mail client like Outlook uses SMTP (Simple Mail Transfer Protocol) to send messages. Later, to download the mails E-mail clients use either POP or IMAP protocol.

But, for any reason, if the authentication fails, mail client like Outlook generates an appropriate error message.

“SMTP Error (535): Authentication failed” error is usually related to the bad user email settings in Microsoft Outlook.

That’s why, Email client configuration always need special attention. Now, let’s see the reasons behind this error and how our Support Engineers correct it for customers.

Causes For the “Error 535: Authentication failed”

From our experience in managing servers, we often see customers experiencing Auth error 535 due to the following reasons:

1. Incorrect Username and Password

When the username and password entered in the Email client are incorrect, it ends up in Error 535. Again, using the wrong the mail server can also cause authentication failures.

Usually, such mismatch in email login and password will be recorded in the mail server logs. This helps greatly in finding the exact email account with problems.

2. Account Disabled

Similarly, at times account can be disabled because of reasons like payment dues or spamming issue. This will also result in SMTP Error (535): Authentication failed.

So, it’s worth to login to the control panel and check the status of the customer account as well as the email account.

3. SMTP Authentication

If your email client do not have SMTP authentication turned ON, it can also result in error. Now, we’ll check on how to enable SMTP Authentication in the server side and client side.

How to configure SMTP Authentication in a Mail client

Turning OFF SMTP Authentication in the email client, show up errors such as: “Server says: SMTP Error (535): Authentication failed: Authentication failure.”

Luckily, configuring SMTP authentication is simple procedure. It involves modifying the SMTP configuration settings and making necessary changes.

For example, to turn ON SMTP Authentication in Mozilla Thunderbird,

  1. Open Thunderbird, go to Tools -> Account Settings -> Outgoing Server (SMTP)
  2. Select the outgoing server by clicking on it, then click the Edit button
  3. Under Security and Authentication, check the “username and password” option
  4. Fill in your email account username and click Ok.

Finally, configuration will show up as below.

Enable SMTP auth in the server

We already saw how to turn on SMTP Auth in email client. But, for this to work, the mail server should support SMTP Authentication. Let’s see how our Dedicated Engineers enable this for our customers.

SMTP Auth in Plesk server

SMTP connections on a Plesk server typically need authentication. Recently, one of our customers requested us to fix the “SMTP Error (535): Authentication failed”. Here, the server was not having SMTP auth turned ON. Therefore, our Support Engineers solved the issue by enabling “SMTP authentication” from his Plesk server.

The exact sequence of steps that we did were:

  1. We logged into Plesk
  2. We checked the SMTP authorization status under Tools & Settings > Mail Server Settings > Relay options.
  3. Finally, we ensured correct settings in Tools & Settings > Mail Server Settings > Enable SMTP service on port 587 on all IP addresses.

Enable SMTP Authentication In cPanel

Let’s have look on the steps to enable SMTP auth in cPanel server. SMTP authentication in cPanel allow sending messages via POP-before-SMTP. Here, once when the Outlook download the mails, there is no need to re-authenticate to send mails through SMTP.

Usually, SMTP authentication will be disabled in WHM by default. To enable SMTP Authentication , we go to WHM >> Service Configuration >> Service Manager and select the Antirelayd checkbox.

[Struggling with SMTP Auth problems? We can fix it for  you.]


In short,“ SMTP Error (535): Authentication failed” error happens when there is a problem with SMTP configuration of email clients. Today, we saw how 1 onlyhost solved “SMTP Error 535 Authentication failed”.

Leave a Comment

Your email address will not be published. Required fields are marked *

Social Share


Cheapest Web Hosting

Fasters Web Hosting Promo
Scroll to Top

Launch your Website at RS599

Create a professional website yourself with our domain hosting combo. Get Extra benefits at this Year end Sale with 10% OFF and Enjoy Amazing Deels on Shared Hosting!

Starting at Only RS599/Year

Year End Sale


Use Code STARTBIZ at Checkout.

Create an earning opportunity with A Web Hosting Business

Become reseller and start reselling domains, web hosting, ssl certificates and More.

45% OFF

Starting at RS399/Month


Use coupon: at the checkout.