Cisco ASA 5500 & Thunderbird

Everthing use to work before installing a firewall at our site (Cisco
ASA 5500). This started happening with one of my users, but now it
happens on my home machine as well:

I launch Thunderbird and it checks for messages. After a minute, it
times out with the server, failing to get any messages. After the
first timeout, everything works fine. Outlook Express and Outlook
2000 both work fine.

If I setup Thunderbird with a different email account (outside our
company), it works flawless.

Our Exchange server (5.5 sp4) is inside the firewall. The end user
has no anti-virus or firewall running on the home pc.

I have tried uninstalling and reinstalling Thunderbird (including
going to an older version).

I have run a packet capture program (wireshark) and the first time it
shows a response of OK from Exchange, but it sends no user info. The
next time, it shows a response of -ERR and then sends user info and
works.

Any ideas?

Re: Cisco ASA 5500 & Thunderbird

Just a little update:

Doing a packet capture on the good side (when TB works receiving
email) shows that TB is sending a CAPA request, and then username,
password, working fine.

The bad time (when it doesn't work the first time), TB does NOT send a
CAPA request, it sends an AUTH request. Then it sends a couple more
packets, and then it sends the FIN to end the connection.