These messages probably also mention an error, likely related to the bounce handling. Something like this: Bounce-afhandeling : Can not authenticate to IMAP server: [CLOSED] IMAP connection broken (authenticate)
I have several Acymailing installs on the same server and dor one of them I receive these kind of messages every once in a while. It's related to a time out or other login problem at the bounce mailbox.
As for the suggestion to send a notification after all the messages in the queue hve been processed, you might want to add that as a feature reuest on Acyba's website: https://www.acyba.com/community/feature-requests.html