What version of Thunderbird?

Is this the vanilla version from https://www.thunderbird.net/ or one provided by your distribution?

It's the version distributed with Ubuntu 20.04. 78.7.1 (64bit)

Make sure to 'accept' the recipients key first. https://support.mozilla.org/en-US/kb/openpgp-thunderbird-howto-and-faq#w_what-is-needed-to-send-an-encrypted-message

https://support.mozilla.org/en-US/kb/openpgp-thunderbird-howto-and-faq#w_what-does-key-acceptance-mean

Hm i'm not sure from the docs how exactly i must 'accept' the key exactly... i already set the trust (see attachment) before all this.

As per your screenshot the key has been accepted.

TB says the recipient has no matching key

Does the recipients email address match the user ID of the key you imported into Thunderbird OpenPGP?

Hi,

yes the address matches the key-identity.

I found out what the problem is: i can only send PGP-Mails if the sender(from:) is the 'main' address of this account. I have sereval aliases for some accounts/identities in TB. Those aliases are all present in the private key as key-identities (listed as 'alternate identities').

How can i send Email with those adresses, that 'share' an account? With enigmail i could do that, and it also should not be relevant unless is want to sign the email, or is it?

You'd need to set up OpenPGP with the corresponding key for each identity.

Oh thanks i found it. I did not look there because with enigmail this did not exists.

For anyone searching: Keys have to be set up here: Account > Other Identities (button below the SMTP settings) > Edit > End-to-end encryption. The main account PGP-key can just be set again there, if it matches the alias