First thing to check:

Thunderbird menu: Tools: Options: Display section: Formatting tab (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu) See my first screenshot below.

Click the "Advanced" button and you should see a window like in my second screenshot below.

In the bottom section for "Text Encoding" make sure that Outgoing Mail is set to "Unicode (UTF-8). This is set by default, but a wrong setting can produce your problem.

Hi Bruce,

Thank you for your elaborate answer. Helas the problem is not solved. The Text-Encoding was already on Unicode (UTF-8). I marked the thick box for answers but as said the error code persists.

Best regards,

Dirk

Try this:

Thunderbird menu: Tools: Options: Advanced section: General tab (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu)

Click the "Config Editor" button.

Search for "mail.strictly_mime".

Change "mail.strictly_mime" from false to true.

Restart Thunderbird and try to e-mail one of those people again. If you get the same bounce-back message, please post that message here, masking personal information.

NOTE: Given the "in process" migration of this forum, this response may not be seen later.

Hi Bruce,

Thanks again for your help. The change of the 'mail.strictly_mime' to 'true' seems to help. I resent some mails that were bounced and uptill now I did not have an error message. If this stays OK I will mark this issue as solved.

Best Regards,

Dirk

Hi Bruce,

The problem has not come back so I consider it as solved. Thanks again for your help.

Best regards

Dirk