Exim Syntax Error – What It Means and How to Fix It

This error usually occurs when an email client (like Outlook, Thunderbird, or the Mail app) sends incorrect or malformed data to the mail server. The most common reason is an improperly formatted recipient address or unsupported characters in the message headers.

1. Quoted Email Addresses

   • Example: '[email protected]' (with quotes)

   • Some mail clients automatically add single or double quotes around addresses, especially when the contact isn’t saved.

2. Invalid Characters in Email Fields
   These may trigger a syntax error during message submission:

   • ❌ Not Allowed:

     • Special characters like $, %, =, ñ, accented letters (é, á, ü), or emojis in headers.

   • ✅ Allowed:

     • Letters (A–Z, a–z, excluding ñ)

     • Numbers (0–9)

     • Periods .

     • Dashes - and underscores _

3. Corrupted Subject or Body

   • Strange formatting, unencoded characters, or invalid MIME types can also trigger this error.

• Ensure there are no quotes or special characters in the “To,” “Cc,” or “Bcc” fields.

• Avoid copying email addresses from sources that may add formatting (like Word or web pages).

• Avoid special or unsupported characters in the Subject or From Name.

• Stick to standard text characters.

• Sometimes the local app may cache an invalid format. Restarting it or using Webmail helps isolate the issue.

• If the server blocks your IP after multiple bad requests:

• Restart your router/modem to get a new IP (for dynamic IPs).

• Or, switch to a different internet connection.

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• If you're unable to send emails due to an IP block, please check for your IP and unblock it from your client area or share it with the hosting support team.

Always add important contacts to your address book to reduce the chance of formatting errors. And avoid copy-pasting addresses from rich-text sources.