Thunderbird itself will never block sending to an email address... but your email provider might. Please quote any error messages (completely and exactly) you see when you try. Similarly, Thunderbird won't block incoming email, though it might mistakenly classify it as spam. Again, please describe what happens to this incoming email that makes you consider it blocked. I do notice that the garmin thing you mention above is not a correctly-formed email address, so it couldn't go anywhere.