No you are not using POP3 - POP3 does not provide access to folders.
You are using IMAP in Aqua Mail. And web mail either also uses IMAP or reads the data files directly.
I am 100% sure that your "trips" and "keepers" folders were created by Aqua Mail *on the mail server*.
And so, please ask your mail provider's support to investigate.
Maybe there is a setting in web mail to "show folders outside the inbox" (
who knows). Maybe it's a "namespace" issue (a mail provider's support should know what this means).
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I think we should reconsider and add a "delete folder" command....