_store_email_flag and _move_email_message (used by the archive / delete / move /
mark-read endpoints) had an else branch that, when _uid_exists returned False,
ran conn.store(uid, ...) / conn.copy(uid, ...) followed by a folder-wide
conn.expunge(). But imaplib's plain store()/copy() take a message SEQUENCE
NUMBER, not a UID, so the op landed on whichever message occupied sequence
position == the UID value, and the expunge then permanently removed it. A stale
cached UID (or a server whose UID probe misbehaves) therefore deleted an
unrelated email instead of reporting 'not found'.
There is no valid case where treating a UID as a sequence number is correct, so
drop the fallback: when the UID isn't present, return False — callers already
surface 'Email not found'. Only the UID command path remains.
Sibling of #1874 (which fixes the auto-spam poller's _imap_move in
email_helpers.py); this covers the user-facing endpoints in email_routes.py.
Part of #2124.
L1