[exim] Non ascii characters in recipient address creates bou…

Hi!
Authorized sender made mistake, used non ascii character in recipient
address. Please look at log (and please forgive me little obfuscation):
2012-11-28 15:40:45 1Tdiof-0007d2-JV <= yyyyyy@???
H=([192.168.2.143]) [192.168.2.143] I=[192.168.2.1]:587 P=esmtpa
A=fixed_plain:yyyyyy S=76559 id=50B62270.1060601@???
2012-11-28 15:40:45 1Tdiof-0007d2-JV ** zzzńzzz@???: Unknown user /
Nie ma takiego uzytkownika
2012-11-28 15:40:46 1Tdiof-0007d2-JV Process failed (1) when writing
error message to yyyyyy@??? (frozen)

Hex values for character "ń" in exim_main.log are: C5 84

I don't understand what happend after rejecting message, at a line with
time 15:40:46. Why exim wanted create bounce message and why process
failed with error code 1?
Thanks, Marcin.