Due to security reason, I have disabled the ICMP ping on my PIX. Afterwards, I find that the Exchange server couldn't route my domain users's e-mail to other Exchange server (hosted in other country). I then review the queue in the Exchange and find that the connector to the other Exchange server is not available. According to my memory, it normally use this queue to route my internal e-mail. However, when I enable the icmp ping on PIX, everything work fine after few minutes.
Two sites are connected through VPN through PIX. In fact, only ICMP is blocked. Other than that, two Exchange could be pinged each other and POP3, SMTP is working fine through the PIX.
Why does this happen ?