Then you should complain to Roadrunner.

Tested by creating and sending directly on Roadrunner Webmail: no problem, so they say it's something in Thunderbird.

Using webmail and sending through a SMTP server are two different things. That result is meaningless. Too bad the RR people are not smart enough to know that. Do you use a signature? If so disable that and try again.

No signature is used. Today Roadrunner "support" guy told me to solve it by sending a message to all my Roadrunner contacts telling them to unblock my messages! Think about that for a minute... send a message to the contacts that Roadrunner won't allow to read my messages!

Most of those help desk people only know to pick up the phone when it makes a noise. Pretty sad.

Maybe you send your message in HTML? Try sending in plain text or in both HTML and plain text to see if that helps.

Check under Tools -> Options -> Composition -> General -> Send Options and/or Tools -> Account Settings -> Composition & Addressing -> Composition -> Compose message in HTML.