Why do I get the message "(killed lost mailbox lock)" user=... host=...?

The traditional UNIX mailbox format or MMDF format is only allows one session to have the mailbox open read/write at a time. So, the email server assumes that if a second session attempts to open the mailbox, that means that the first session is probably owned by an abandoned client. The common scenario here is a user who leaves his client running at the office, and then tries to read his mail from home. Through an internal mechanism called kiss of death, the second session requests the first session to kill itself. When the first session receives the "kiss of death", it issues the "Killed (lost mailbox lock)" syslog message and terminates. The second session then seizes read/write access, and becomes the new "first" session.

Certain poorly-designed clients routinely open multiple sessions to the same mailbox; the users of those clients tend to get this message a lot.

Another cause of this message is a background "check for new mail" task which does its work by opening a POP session to server every few seconds. They do this because POP doesn't have a way to announce new mail.