ActiveSync doesn't sync new Inbox messages

We have Exchange 2003 with service pack 1. For antivirus protection we use
Sophos Mail monitor. Using ISA server 2000 we published OWA and ActiveSync
using SSL. OWA works just fine. But with ActiveSync I have very wired
behavior.

When I try to sync Inbox on my Pocket PC (Windows Mobile 2003 on Compaq
3970) with Exchange everything are synchronized except new messages that I
received. But when I open that new message in preview pane using OWA, then
messages are synchronized.

So I received some new messages. Synchronized PocketPC and nothing (no new
messages). Then I open messages using OWA and then synchronized again. And
then new messages are synchronized.

I try verbose logging on PocketPc and everything look just fine. Also
<estimate>e-mail _number<estimate> wbXML tag in PocketPC log show right
number of messages.

Any suggestion?

Tnx.