Have you checked whether anyone else is able to send an email to this address? Like ask a friend to try?

The error message - at face value - says that the email account isn't accepting any email traffic.

Only I have this problem. Other my team mates sending mails to the same addresses without any problem. Me too, but only when I don't use Thunderbird.

I'm stumped but, as this is a Bulgarian company with it's own alphabetical script, is it possible that there is a special character in the email address that looks the same as the normal English one?

And, what happens when you reply to an email sent by this email address? Does that work?

Just a couple of things to eliminate before the pros join the thread :)

With other mail client have not any problems with the same addresses. When use reply, problem is the same

Okay. I'm out of suggestions... ...except he might have accidentally kill-filed you?