I'm wondering if somebody else has experienced similar problem with ASA 5520. We have an e-mail server on inside and spam filter on outside. I have allowed smtp from spam server to e-mail server on inside and www from any outside to e-mail server on inside for OWA plus pop3. I was able to send e-mails, but not to receive. OWA connection was just timing out. It was possible to connect, but in a couple of second the connection was gone. I checked logs on ASA and I'm sure that issue is not the access list. Connection to another server on the same subnet was fine, so the problem wasn't routing either.
I got rid of esmtp inspection and got the same result. The server in question is performing badly, possibly this problem would not occur with a most of regular servers. I'm really curious what possible work around could be others than changing the server.
Thanks for sharing any ideas