This issue arises due to Cisco bug ID CSCse47150 in PIX Firewall version 7.2.1.
The presence of another bug CSCsd72617 can also trigger the same issue.
The PIX Firewall/Adaptive Security Appliance (ASA) running software version 7.2.1 can crash with the Thread name Dispatch Unit error when the Extended Simple Mail Transfer Protocol (ESMTP) inspection engine is enabled.
The issue canarise while you process segmented SMTP/ESMTP packets.
After an upgrade to software version 7.2.1, the PIX Firewall can continuously reboot when the ESMTP inspection engine is enabled. By default, ESMTP inspection is enabled.
The problem occurs when the data length for segmented packets are miscalculated.
As a workaround for this issue, complete either one of these steps:
Disable the ESMTP inspection engine.
Apply the no inspect esmtp command in the PIX /ASA global configuration as shown in this example:
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