Помогите плз. со всеми почта нормально ходит, а от одного ящика возвращается. Пишет :

The following addresses had permanent fatal errors -----
<khs@khs.kiev.ua>
    (reason: 450 Client host rejected: cannot find your hostname, [212.109.61.234])

   ----- Transcript of session follows -----
... while talking to smtp.khs-ag.com.:
>>> RCPT To:<khs@khs.kiev.ua>
<<< 450 Client host rejected: cannot find your hostname, [212.109.61.234]
<khs@khs.kiev.ua>... Deferred: 450 Client host rejected: cannot find your hostname, [212.109.61.234]
Message could not be delivered for 5 days
Message will be deleted from queue

В чем может быть проблема. у меня или у этого ящика?
Rh7.3