What Does "Mailbox locked" in the Delivery Logs Mean?

When reviewing the delivery logs for an installation of SmarterMail, you may see messages that say "Mailbox Locked."
 
This message appears if SmarterMail tries to deliver a message to a mailbox that is currently being accessed via POP. If this is the case, the messages that are waiting to be delivered to that mailbox will remain in the spool until such time that the POP session is completed. Once that happens, the messages will be delivered to the mailbox.