[exim] Sporadic "Unroutable main domain" error

Hallo!

I run Exim 3.35 a small mailserver (static IP, "always on" since years)
since years, and everything went fine. Until last week, when outgoing
mails are bounced with the reason "unroutable mail domain".
When the users send the mail again after getting the bounce-message, the
delivery succeeds.

I first thought there were some DNS problems, as I run BIND9 for
name-solving with some forwarders. I thought maybe the forwarders were
too busy to reply in time or so. So I removed them to run the hierarchy
myself, but that did not solve the problem.
I thought that could be the BIND9 reloads its config one hundred times a
day (customers logging in updating their entry in the zonefiles -
dynamic DNS) - but in the logfiles there is no connection between the
exim-errors and the BIND-reloading (timestamp in mainlog and daemon-log
differs up to 12 minutes).


By googling I found there were some other people with a similar problem,
but without a solution. Most I found has to do with reproduceable
problems, but mine seems not to be.
So I ask here what can cause this error message and maybe how to get rid
of it.



Saluti!

Ludwig