[exim] Spool file is locked (another process is handling thi…

Hi,

I am relatively new to exim and have a big problem.
I am using Exim 4.50 on Debian Sarge on my personal mail server which is a virtual server. It is
the exim4-daemon-heavy package.

Everything works fine for some months now, but since last week I am unable to send mails to one
specific guy. When I type "mailq" I see these mails which do not get send. I am able to send mail
to any other address I have tested.

The mainlog says:
2005-09-29 02:09:29 1EKeSt-0001ZE-Sw Spool file is locked (another process is handling this message)

which is of course repeated every 30 seconds.

Does anybody have any idea what this may be? I cannot think which other process this could be. I
use clamav and spamassassin which are scanning incoming messages but these are outgoing messages
and any other receipient works.

Thanks for any help,

Oliver