Since Bug 171073 has been fixed, the filter engine is now capable of processing arbitrarily complex Boolean expressions. There are two problems though: 1) the UIs have not been updated to allow users to take advantage of this feature. The feature is only used internally, for managing virtual folders. (There is already Bug 274192 requesting that the Search UI be updated, but obviously the Filter UI must get a corresponding update.) 2) the current Filter expression language (i.e., what gets saved in msgFilterRules.dat) is ill-suited to actually representing complex filters. On bug 171073 comment #13 I made a suggestion to borrow the operator syntax used for the string representation of LDAP search filters. We could use anything really, but standardizing on LDAP format may allow consolidation of this code with Address Book searching, for example.

*** This bug has been marked as a duplicate of 297852 ***