You can only "Customize From Address" if the domain you're wanting to send from is added to your sending profiles. So. Yes. it's possible. But it's user-hostile.
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.
No I mean, I have many different domains.
So if I want to send an email from example.com, I have to add it to the sending profiles. This is fine, and usually done during initial setup. However, if I want to be able to send from a second domain, say, thisdomaindoesntexistasdadasd.com I need to go and add that seperatly for allow thunderbird to allow me to put it in the from addr. Further, when sending an email, I then have to select the domain from the drop-down, then edit. it's just tedious. Not impossible to work around, but also not worth the effort when the Fastmail UI handles it so much better.
This has been possible for years. Click the From address then "Customize From Address..." and enter whatever you like. But
> And TB still doesn't "remember" which -from- addresses are used by which -to- address.
is indeed correct, and annoys me too.