Why Gmx mail does not support Co

GMX Mail's issues with the Content-Transfer-Encoding field can be traced to several factors:

1. Server-Side Processing: GMX may have specific rules or configurations on their mail servers that lead to issues with the Content-Transfer-Encoding header. These configurations can sometimes cause problems with encoding certain types of email content, particularly attachments (Hestia Control Panel - Discourse).

2. Email Client Compatibility: Some users have reported issues with email clients like Thunderbird not displaying attachments correctly when receiving emails from GMX accounts. This suggests that GMX's handling of MIME parts and encoding headers might not always align perfectly with how some email clients process these headers(MozillaZine Forums).

3. Error Handling and Syntax Issues: In some cases, GMX's mail servers might reject emails or misinterpret them if they detect syntax errors or unsupported parameters in the headers, including the Content-Transfer-Encodingfield (Hestia Control Panel - Discourse).

To mitigate these issues, users might need to:

• Ensure their email clients are up-to-date and configured correctly.
• Use alternative encoding methods if possible.
• Consult GMX's support for specific configuration settings that might help resolve encoding issues.