Update after direct authenticated IMAP testing (2026-08-11): the mailbox exists and STATUS reports 0 messages. DELETE "Cestino/pippo" returns BAD [CANNOT] DELETE failed - Mailbox is reserved. Renaming it to a temporary root mailbox succeeds, but DELETE on the renamed mailbox returns the same reserved-mailbox error; the original name was restored. UNSUBSCRIBE returns OK, but an immediate LSUB still reports the mailbox as subscribed. This confirms a server-side reserved orphan mailbox that clients cannot delete or unsubscribe from.For more than a year, Yahoo IMAP has continuously reported the folder "Cestino/pippo" (Trash/pippo). All Thunderbird clients synchronize it, while Yahoo Mail on the web does not display this subfolder. It currently contains 3 unread messages. The account has 0 Yahoo filters, and the active Thunderbird profile has no local filters or settings that reference "pippo". Removing or hiding it locally does not resolve the issue because it returns from the Yahoo IMAP server. Please investigate this server-side orphan mailbox and provide a safe way to remove it without losing messages.