If you are using a mail app like Outlook, Thunderbird, or Apple Mail, and suddenly your emails will not load and you get an authentication error when trying to send mail, you are not alone!

This is one of the most common issues we see, and it is usually tied to an automatic security guard on our hosting servers. Let's look at exactly what is happening behind the scenes, what the technical error means, and how to fix it right now.

Why Am I Getting an "Authentication Failed" Error?

To keep your mail secure and keep hackers out of your inboxes, Rebel's Plesk email servers run an advanced security shield called Imunify360.

If a device on your local Wi-Fi network repeatedly tries to connect using an outdated or incorrect password, our server logs a specific technical event:

"SASL PLAIN authentication failed: authentication failure"

Because the server sees a device trying to log in with the wrong password over and over again, the security system assumes it is a hacking attempt. To protect your business, the firewall automatically blocks or "greylists" your entire office network's IP address.
 

The Tell-Tale Symptom:

• Your mail apps will stop syncing, and you will get an "authentication failed" or "failed to connect" error when sending or receiving.
   

• The Webmail Loop: The only way to make it work is to open a web browser and go to your Rebel Webmail page. Once you pass the verification screen in your browser, your computer's mail app suddenly downloads all your messages instantly.

What Device is Causing the Error?

Usually, the "bad login" attempts are coming from your own home or office! Common culprits include:

1. An Old Device: A tablet, old phone, or spare laptop where you recently changed your email password, but forgot to update it.
    

2. Office Hardware: A smart printer, copier, or scanner set up to "Scan-to-Email" using an old password.
    

3. Background Cache: A mail client running in the background with stale credentials saved in its settings.

How to Resolve This Right Now

• Step 1: Get Whitelisted

  Reach out to the Rebel Support Team and let us know your emails will not load due to an authentication error. We will grab your local IP address and whitelisted it on the server so you can send and receive immediately.
   

• Step 2: Hunt Down the Failing Device

  A whitelist is temporary. To prevent the server from blocking you again, locate the device on your network that has the wrong password saved and update it.
   

• Step 3: Reset Your Saved Password

  In your active mail client, delete your saved passwords, restart the app, and type in your password fresh.

If you aren't sure which mailbox is failing, ask us! We can check our Plesk logs, tell you exactly which username is throwing the SASL PLAIN error, and help you get it sorted out.