SMTP Authentication is the process by which an ISP’s clients authenticate themselves with the mail server to which they intend to send an email. No one can send email via any mail server; mail servers will only allow legitimate users to send an email.

How it operates:

When a user sends an email, the user’s mail client (Outlook, Outlook Express, Thunderbird, etc.) authenticates itself to the mail server by providing the email account’s username and associated password. If these credentials are valid, the user’s mail is accepted and processed before being relayed to the intended recipient. If authentication fails, the user’s mail client generates an appropriate error message.

What Causes “Error 535: Authentication failed”?

1. Incorrect Username and Password

When the username and password for the Email client are entered incorrectly, Error 535 occurs. Using the incorrect mail server can also result in authentication failures.

Typically, such inconsistencies in email login and password are logged in the mail server’s logs. This greatly aids in locating the specific email account that is causing problems.

2. Account Disabled

Similarly, accounts may be disabled for a variety of reasons, including past-due payments or spamming. Additionally, this will result in SMTP Error (535): Authentication was unsuccessful.

Thus, it is worthwhile to log into the control panel and check the status of both the customer and email accounts.

3. SMTP Authentication

Additionally, if your email client does not have SMTP authentication enabled, this can result in an error. We’ll now examine how to enable SMTP authentication on both the server and client sides.

How to configure SMTP Error 535 Authentication Failed in a Mail client

When the email client’s SMTP authentication is disabled, errors such as “Server says: SMTP Error (535): Authentication failed: Authentication failure” appear.

Fortunately, configuring SMTP authentication is a straightforward process. This procedure entails modifying the SMTP configuration settings and making necessary modifications.

To enable SMTP authentication in Mozilla Thunderbird.

1. Navigate to Tools -> Account Settings -> Outgoing Server in Thunderbird (SMTP)
2. By clicking on the outgoing server, you can select it and then click the Edit button.
3. Select the “username and password” option under Security and Authentication.
4. Enter your email account’s username and password and click OK.

Enable SMTP Auth in Plesk server

1. Log into Plesk
2. In Tools & Settings > Mail Server Settings > Relay options, verify the SMTP authorization status.
3. Finally, ensure that all IP addresses have the correct settings in Tools & Settings > Mail Server Settings > Enable SMTP service on port 587.

Enable SMTP Authentication In cPanel

Let’s take a look at the steps required to enable SMTP authentication on a cPanel server. In cPanel, SMTP authentication enables the sending of messages via POP-before-SMTP. Once Outlook has downloaded the emails, there is no need to re-authenticate in order to send emails via SMTP.

SMTP authentication is typically disabled by default in WHM. To enable SMTP Authentication, navigate to WHM >> Service Configuration >> Service Manager and check the Antirelayd option.

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