Your recipient's mail host (server) is rejecting messages addressed to that address. They need to contact their mail host and find out why that is the case. This is not something you can fix from your end or in Thunderbird.

Your recipient's mail host (server) is rejecting messages addressed to that address. They need to contact their mail host and find out why that is the case. This is not something you can fix from your end or in Thunderbird.

Thank you so much for your answer. I wrote to them in the first place and got the answer: "From the screenshot it looks like you are not responding directly to Allegro - but to software, ads or other software that reflects messages sent by you". I replied directly to messages I received from them. So I have to write to Allegro customer service again to fix the error?

I don't expect them to readily acknowledge something is amiss on their end. They probably know all too well what's really going on on their end, say overzealous anti-spam filtering deployed on their server, but they would rather not accept this to be the case, so instead, they send you on a wild goose chase. See https://spolecznosc.allegro.pl/t5/zaawansowani-sprzedawcy/i-jak-tu-pisać-do-allegro-kontakt/td-p/109076