📬 What Is a Protocol Mismatch?

Email accounts use specific protocols to communicate with mail servers. The two most common incoming mail protocols are:

• IMAP — Syncs emails across all your devices. Emails stay on the server.
• POP3 — Downloads emails to a single device. Emails may be removed from the server.

A protocol mismatch occurs when Mailbird Next is configured to use one protocol (e.g., POP3) but the server expects the other (e.g., IMAP), or when the port number does not match the selected protocol.

⚠️ Error Reference

If you see Error #21 in Mailbird Next, it typically indicates that the server rejected the connection because of a protocol or port mismatch.

🔍 Common Causes

• Using a POP3 port (110 or 995) with an IMAP configuration, or vice versa.
• Using a non-SSL port when the server requires SSL/TLS, or the reverse.
• Your email provider has disabled POP3 and only supports IMAP (common with Gmail, Microsoft 365, and others).
• Incorrect server hostname for the selected protocol.

🛠️ How to Fix It

1. Check Your Server Settings

Go to your account settings in Mailbird Next and verify the following:

1. Click the Settings icon (gear) in the bottom-left corner.
2. Go to Accounts and select the affected account.
3. Check the incoming server settings:

Make sure the port number matches the protocol you have selected.

2. Check Your Outgoing (SMTP) Settings

3. Verify with Your Email Provider

If you are unsure which settings to use, check your email provider’s support page for the correct server addresses, ports, and protocol type. Most providers publish this information publicly.

4. Switch to IMAP (Recommended)

If your provider supports both protocols, we recommend using IMAP for a better experience. IMAP keeps your emails synced across all devices and is the modern standard for email access.

💬 Need More Help?

If you’re still having trouble with protocol or port settings, please contact our Customer Happiness Team. We’ll be happy to help you get connected.