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Try to increase the mail inbound polling.
Perhaps is you have 1 Minute in place, the third mailbox is completed and the fourth would be next, but because of 1 Minute Polling interval the service starts from the beginning again.
Try 5-10 Minutes and see what happens.
I took screen shots and put all 5 side by side, two used port 995 and not 993. I am leaving the polling time at 5 minutes because I am sure it won't hurt. You were right though, I did have it set to 1 minute.
There aren't any limits on this and the poll period shouldn't matter either. The first thing to check is the event log on the app server to see if the inbound mail service is generating any errors. Also restart the service to make sure it has picked up the latest configuration changes.
See this doc for information on adding more verbose logging if required: Troubleshooting Inbound or Outbound emails using Log Severity.