What do you mean you cannot email that email address? What happens if you send an email to it from some other address? I would normally ask you to post the server settings you're using in Thunderbird to check whether they're correct, but the problem here appears to be the username itself (i.e. the email address), that doesn't exist…

Hej igen. Alla inställningar är lika på alla e-postkonton jag har men det är bara det här kontot som inte fungerar och jag får upp det angivna meddelandet. Jag har försökt att skicka meddelanden till kontot från andra konton men det fungerar inte då heller.

Well, the problem appears to be that an account with that username (email address) doesn't exist, so if that's the case you won't be able to set up the account in Thunderbird even if you're using the right settings. Have you tried logging into that account using webmail?