Question: I am displaying calendar using calendar control.

Each date is displayed as Link button.

But i want only last 30 days(dates) from current date, to be enabled for client click. on those days i will write some code on SelectionChanged event. But goal is other dates must be disabled for click.

They should not have link button.

Answer: 
Because POP3 does not need as much storage on the server side. And because it is more simple and easy to implement.
A few reasons:

1. POP3 does need far less server side
storage.
2. POP3 is much easier to implement.
3. POP3 is private by default (it
assumes that you want to delete
email off the server after
downloading it).
4. Initially almost all ISPs had tiny
storage limits (since their are free
large storage email services this
matters less now) so deleting emails
that you downloaded was a must.
5. (for completeness - thanks Evan) POP
clients open a TCP/IP connection to
the server, authenticates itself,
asks for any email and does whatever
processing it needs, and then close
the connection. IMAP clients
generally keep one or more TCP/IP
sockets open with the server the
whole time they are running, taking
up those resources.
6. POP is a much simpler protocol to implement and can handle a greater number of clients with the same amount of resources.