mutt_dotlock(1) on Linux
The text e66f9f6e0644, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it. The page shows this text now.
Lock mail spool files.
Flags it lists
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-d -
-f -
-p -
-r -
-t -
-u