POP is the mechanism to using most of the people on the internet use the get their mail. now currently time POP3 version , RFC1939 defines this protocol. It is a supporter to support to single mailbox for each user. Actually more confusion occur to source, Much of what people are familiar when using POP. There are more mail message they have are not supported by POP at all. this capability are inbuilt into the program like as Microsoft Outlook. So, when using the JavaMail API, if you want this type of information, you have to calculate it yourself.