Lotus Domino and mail routing problem

I’ve a client who have problems with replying e-mail. Some of the mail returns with followin return receipt (Names have been changed) :

cc: Postmaster@inet
Subject: Returned Mail: Message Could Not Be Delivered

From:User.Name/OU=notes/DD.hpnotes=#b#Johanna#b#Helstela#b##a#company@naakka
550 Johanna.Helstela/OU=notes/DD.hpnotes=#b#Johanna#b#Helstela#b##a#company@naakka …. Message unable to be mapped at gateway

Everything seems to be OK at our server but still this message appears from time to time. Where is the problem?