Tapping internal mail?

Anyone ever come across trouble tapping emails within the same domain?

A domain wide tap works just fine mind you. However a single address tap only works if the From is not from the same domain as to To. For internal mail, the rule is simply skipped and the domain-wide tap catches it instead.

Have looked at qmailtoaster patch 1.3.2 but I can't see any explanation there (not surprising mind you given I'm not familiar with qmail's source and not a C coder either).