Hmm. Do I have it right that you mean you can't send from e.g. "test@example.com" (using "Customize From Address..." then entering that address in the From field) unless another address for the domain "example.com" is set up as an account in Thunderbird (e.g. "hello@example.com")? If that's what you mean, then I don't think that's correct, as I just tried it and Thunderbird let me send the message despite me not having any account for example.com in my profile. I did of course get a response from example.com telling me there was no MX server, but that tells me Thunderbird went as far as sending it.