You may see the following error in SmarterMail delivery logs:
Delivery attempt to email@example.com to folder ‘Inbox’ failed. Mailbox is 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.
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