[Exim] Virus | Spam tagging

Hi

I am using exim4 for some time on debian sarge now and I works fine.
My configuration is totally virtual, no linux users ever involved.

Now I want to start doing spam and virus scanning and flagging.
By first target is to tag mails only.
Detected spam should get [SPAM] added to the subject.
Detected viruses should get [VIRUS] added to the subject.
For the beginning I do NOT want to deny, reject or delete any mail.

Furthermore, the most important point is stability. exim may not fail.

Can someone tell me where to start or even give me some config-snippets?

I read a lot about clamav, which I have installed, but I also read, that it
could fail multiple times a day. In that case, exim would fail also. This is
inacceptable.
I also read a lot about spamasassin, do I have to risk the same here?

I would not mind, that in case of any problems mail would get delivert
without any scanning.
But i would mind, if may would not be delivered.

I would like to not pass the mails around by smtp between exim and
av/spamdetectors. I would prefer exiscan or pipelineing if possible.

By the way, there is about no information in the list about AMAVIS?

I hope for any hints...

Thanks
Steffen