I have not used Opera Mail, as yet, but I do know
there are two ways in which any E-Mail client can use
such folders. Thunderbird creates a virtual folder
which tags the message, and makes it appear as though
it is in a separate real folder/directory. The Bat
creates true folders/directories to place those
filtered messages. How do I know this? No separate
folders, except for the regular folders which are
hard-programmed, can be found, when checking out
Thunderbird. You can easily spot the folders created
by and for The Bat.

Hope I have been helpful.

Regards,

Julian.