That error message is not Thunderbird. It is a message from the server and it is blocking you.

Spam like characteristics have triggered the server Spam filter. good general info: https://knowledgebase.constantcontact.com/articles/KnowledgeBase/5649-common-phrases-that-trigger-spam-filters?lang=en_US

You need to check the content of the message you are sending. Does it have too many links or other remote content ? Do you have a signature in the email ? Test by removing the entire signature and then try sending.

Are you sending in both Plain Text and HTML? Check you are sending in both. Menu app icon > Options > Composition Under 'HTML Style section click on 'Send Options' Select 'Send the message in both plain text and HTML'

See if your IP address is causing an issue: https://www.spamhaus.org/lookup/

Thank you very much for your support. This afternoon when I got your reply I tried again with a simple message, 1 word 'test', to my self, and it did not work. And have asked my friends, that are not using THB, having no problems sending mails with the same server. I have not been able sending mails since Mars 1, and ver 78.8.0 was installed Mars 1, so that's why...

Now, this night I should try your advice, but I first test send the 'test'-message, AND it WORKS, I have changed nothing !!??

The server company must have done something, I do not know yet...??

THANK YOU for your support

Regards Gunnar Lagerlof SWEDEN