lockspool — lock user's system mailbox

lockspool  is  useful  for  a  client  mail  program  to  attain  proper  locking.   lockspool  obtains a
       username.lock for the calling user and retains it until stdin is closed or a signal like SIGINT, SIGTERM,
       or SIGHUP is received.  Additionally, the superuser may specify the name of a user in  order  to  lock  a
       different mailbox.

       If  lockspool  is able to create the lock file, “1” is written to stdout, otherwise “0” is written and an
       error message is written to stderr.  lockspool will try up to 10 times to get the lock  (sleeping  for  a
       short period in between tries).

       Typical  usage  is  for  a user mail agent (such as mail(1)) to open a pipe to lockspool when it needs to
       lock the user's mail spool.  Closing the pipe will cause lockspool to release the lock.

       The lockspool utility exits 0 on success, and 1 if an error occurs.

/var/mail/username.lock  user's mail lock file

       The lockspool program appeared in OpenBSD 2.4.

Debian                                          January 25, 2019                                    LOCKSPOOL(1)

       lockspool — lock user's system mailbox

mail(1), mail.local(8), smtpd(8)

lockspool [username]