The default POP3 server installed on iSeries boxes allows for username enumeration. This PDF contains a table converting POP3 login errorcodes to their actual meanings.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/37224/Enumeration_of_AS400_users_via_pop3.pdf
Source: https://packetstormsecurity.com/files/37224/Enumeration_of_AS400_users_via_pop3.pdf.html