I have two different confirmed occurrences of this, in both cases when doing an initial sync of a POP3 mailbox with tens of thousands of messages, the POP3 resource only syncs a thousand or so in a single go. Multiple syncs must be run manually in order for the POP3 resource to fetch all messages.

I haven't investigated why this is, it could be a limitation in POP3 servers implementations, could be some sort of connection timeout but could also be an issue on the Resource side (KIO?)