I have supported the product for many years and do not recall any such issue being reported. The problem you now face is that the product is End of Life. Thus you might now want to consider upgrading to the new product which incorporates this technology and includes new features. The new product name is McAfee Security 7.5 for Lotus Domino which has an Antispam add-on component.
If the problem still occurs after an upgrade which I believe would be unlikely, would mean McAfee Technical Support would be able to investigate. However they would need an SMTP RAW file to test with. This could be used by us within a TELNET session on a test server to reproduce the issue. In addition to this any case that needs to be escalated to the next level would also need a set of Minimum Escalation Requirements (MERs).
I am sorry on this occasion that I cannot point you to an article that covers this issue.
End of Life Document:
KB60584 - GroupShield 5.3 and SpamKiller 2.1 for Domino End of Life statement
KB59385 -Minimum Escalation Requirements (MER) tool list for McAfee Security Products