What does it mean when "Domain filters passed illegal address (how'd that happen?)" is logged?

Issue

What does it mean when "Domain filters passed illegal address (how'd that happen?)" appears in the system log?

Solution

This is just a warning from the content filtering implementer that it is processing a non-RFC-compliant email address. This is either the result of an invalid address being specified in the filter syntax or (more commonly) an illegal address specified in the message which wasn't flat out rejected by the SMTP agent.

While address-specific filters might not parse the address correctly, filtering should continue and this shouldn't cause side effects for other filtering implementers or other messages.

Error Log Might Contain the following :

MMFI: WARNING Message Filtering

MCF: Domain filters passed illegal address (how'd that happen?):

Error Log Sample:

2010 Sep 13 04:51:54 gol-mas2 mmfid[35586]: MMFI: WARNING Message Filtering (QID GQS22655, message ID <201009121851.GQS22655@austarnet.com.au>): MCF: Domain filters passed illegal address (how'd that happen?): 11568576707AUVV@SIHwiVHxhLVomUAoHw@uj75.tyydeal.com