You could technically use an IMAP server and client as a Kanban board. Make a folder per swim-lane, make a backlog, move e-mails from one folder to another as you get each item done, share the mailbox with your team. IMAP already supports searching, filters, flags, attachments. You can treat a thread of replies as comments on the original item and move them all as a group. Create a custom client to hide the fact that the backend store is basically an immutable log journal of MIME entries.