To get to this page, to to Services → POP3 Server → Advanced

The Implement fix for Microsoft Internet Mail build 1160 option is to implement a workaround for a bug in a VERY old version of Microsoft's free email client (the precursor to Outlook Express). There is no harm leaving this option on, but it should equally cause no problems to turn it off nowadays because it is very unlikely that Microsoft Internet Mail is still being used (it was replaced by Outlook Express in 1997). This option removes blank lines from the end of the message.

The Hold messages instead of deleting them option tells VPOP3 that when the POP3 email client deletes a message, VPOP3 should 'hold' the message instead. Held messages are invisible to email clients so this will make the message appear to have been deleted as far as the email client can see, but it will still be present on the server. Note that there is no automated 'cleanup' option in VPOP3, so if you enable this option, the mailbox size in VPOP3 will grow indefinitely.

The Max failed login attempts option tells VPOP3 how many failed login attempts to allow in a single session before the connection is dropped and the client will have to reconnect.