We updated Avast to the latest version on a client's server running SBS 2011. The update apparently included antispam. All of a sudden, the Transport service kept failing, and all messages (in both directions) landed in the Poison Queue. Restarting the Transport service did not fix the issue, and there was nothing we could do.

Googling around, we found that the trouble was indeed related with Avast. Disabling the Antispam Shield did not resolve the issue, but disabling the transport agent in Exchange, via Powershell was the fix:

disable-transportagent "avast antispam for exchange"

After disabling the agent, everything came to life again.

@ Avast: you should really start beta-testing things better. Other products have similar deficiencies, and this just won't do on production environments. Get your act together or start losing a lot of clients.