[exim] Question about "defer (-23): domain is held"

Dear exim users,

I'm new to exim and I see following message in exim log file repated for
one my local domain defer (-23): domain is held.
That worries me.

Although i searched for possible reasons using google I found no
relevant answers.
Could someone explain me how to solve this?
I use exim4.

--
Zbynek Houska <zbynh@???>