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.
Another reason is POP3 fetchers. You may have a central IMAP mailbox where you want to download mail from all other accounts of yours to keep all your mail in same place. This is usually done with POP3 fetchers which poll your other mailboxes periodically and load mail from there in your primary mailbox.

Although IMAP fetchers do exist, most of fetchers are still POP3-only and if your other mailbox does not support POP3, you may have troubles using a fetcher to link it with your primary mailbox.