Poco General FAQ - Searching for marked messages
I know you can use Show marked to view only marked messages in one mailbox, but is it possible to search all mailboxes for messages that have been marked?
In Poco 3, this option is built into the Search feature.
You can search for the "X-Mark" header, so searching for "X-Mark: 2" would find all messages marked with second marking. You have to keep the space in front of the number for this to work well.