I don't see the + option, attached screenshot
This option is only available with IMAP (and Exchange) accounts since the POP 3 protocol does not provide functionality for creating folders or managing email in folders. Therefore, it is not possible for a POP3 client to provide this functionality.
Please read the notes below:(1) Mail clients such as Outlook or Mozilla Thunderbird download email from POP3 and manage email in folders on your local computer.
(2) Many mail servers implement both POP3 and IMAP protocols. Microsoft Exchange Server is one such mail server. In these cases, it is possible to create and manage folders via the IMAP protocol.
(3) Some POP3 mail servers provide the ability to create sub-folders. However, access to those folders is specific to the brand of mail server. Also, the creation an management of the sub-folders is not available in a programmatic way, and certainly not via the POP3 protocol (because there are no POP3 commands related to folder management).