How to Fix "Invalid Certificate" or Security Warnings in Email Apps (Plesk Hosting)

If your email app (such as Outlook, Apple Mail, or Thunderbird) displays a warning like "Invalid Certificate", "Untrusted Server", or "SSL Certificate Expired", the issue is almost always caused by an outdated mail server name saved in your app's settings.

Why Am I Seeing This Warning?

Many email apps were originally configured using older server names (such as canada1.rebel.com or canada3.rebel.com). While these legacy names still route your email behind the scenes via internal proxies, their security certificates are no longer renewed.

When your email app connects, it notices that the security certificate does not match the old server name, causing it to pop up a security warning.

Who Needs to Update?

Email Apps (Phone, Tablet, Desktop): ACTION REQUIRED. You must update the incoming and outgoing server names in your email app.
Webmail Users (Browser): NO ACTION NEEDED. Webmail (webmail.yourdomain.com) updates automatically in your browser.

How to Fix the Certificate Error

To permanently clear the security warning, update your email app settings to point directly to your domain's dedicated mail server address:

===================================================================================
                       REQUIRED SERVER SETTINGS UPDATE
===================================================================================
OLD SERVER NAME (Deprecated):   canada1.rebel.com / canada3.rebel.com (or similar)
NEW SERVER NAME (Required):     mail.YOURDOMAIN.TLD (e.g., mail.example.org)
===================================================================================

Step-by-Step Instructions:

Open your email app settings on your phone, tablet, or computer.
Go to your account's Incoming Mail Server and Outgoing Mail Server (SMTP) settings.
Replace the old server name (e.g., canada3.rebel.com) with mail.yourdomain.com (replace yourdomain.com with your actual domain name, such as mail.example.org).
Ensure SSL/TLS security is enabled:
- IMAP Incoming: Port 993 (SSL/TLS)
- POP3 Incoming: Port 995 (SSL/TLS)
- SMTP Outgoing: Port 465 or 587 (SSL/TLS)
Re-enter your email password in both the incoming and outgoing password fields.
Save your changes and restart your email app.

Additional Troubleshooting

1. Refresh Your Server's SSL Certificate (For Hosting Account Admins)

If mail.yourdomain.com continues to show a certificate error after you update your email app, refresh the free Let's Encrypt SSL certificate directly inside your Plesk control panel:

Log into your Plesk Hosting Dashboard.
Go to Websites & Domains > SSL/TLS Certificates.
Click Re-issue Certificate (or Install a basic free certificate provided by Let's Encrypt).
Make sure the box labeled "Secure mail containing the domain (mail.yourdomain.com)" is checked.
Click Get it Free to install the updated certificate across your mail server.

2. Re-adding Your Email Account (Important Warning!)

If updating your settings does not clear the cached security prompt, you may need to delete and re-add your email account in your app.

CRITICAL WARNING BEFORE DELETING ANY EMAIL ACCOUNT:

IMAP Accounts: SAFE TO REMOVE. All your emails, folders, and sent items are saved on the server. When you re-add the account, everything will sync back automatically.

POP3 Accounts: DO NOT REMOVE WITHOUT A BACKUP! POP3 downloads emails directly to your specific device and deletes them from the server. Removing a POP3 account from a device (especially an iPhone or iPad) will permanently delete all stored emails.

Need Further Assistance?

If you continue to experience security prompts after updating your incoming and outgoing mail servers to mail.yourdomain.com, please contact our support team.

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