How does IMSS process a partial email messages?

IMSS rejects partial email as a malformed message if BypassMessagePartial=no in the imss.ini file (default setting).

If the key is set to yes, IMSS will bypass the partial mails. Trend Micro does not recommend changing the item BypassMessagePartial to yes as this may cause virus leak.

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How does IMSS process a partial email messages?