The Mail Pop server preference stores the URl of the POP3 server to be checked for email. Default is empty - no value.
Note, some ISPs may have different mail server addresses for POP3 and SMTP servers - make sure you use the address of the POP3 server.
Tinderbox assumes the POP3 service is on the standard POP3 port - #110. Mail providers requiring access on port numbers other than 110 are not supported.