Client host rejected: cannot find your hostname

[HTR]

Hi guys,

need your help.
Every day I receive automatic emails from the domain (the domain was changed for obviously reasons, ok) abcd.efg.hi, but today, I've checked zimbra.log and I've seen this:
NOQUEUE: reject: RCPT from unknown[xxx.xxx.x.xxx]: 450 4.7.1 Client host rejected: cannot find your hostname, [xxx.xxx.x.xxx]; from=<user@abcd.efg.hi> to=<me@xxx.xxxx.xx> proto=ESMTP helo=<mail2.abcd.efg.hi>

What's happening? I suppose that the problem is with the guys at the domain abcd.efg.hi, isn't it? Maybe the reverse.

Thanks for any help.