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.
I think part of it is that IMAP is usually used in scenarios where the emails are retained on the server, whereas POP is used almost exclusively for situations where the email is downloaded to the local machine, which is what most ISPs providing email facilities for their customers want you to do (and also what most of the customers probably want as well, although less so as they get more exposure to the Internet and things like gmail and yahoo, etc.)

Another reason it is probably preferred by email providers is that it does polling, much like an RSS news reader. The POP client will open a TCP/IP connection to the server, authenticate itself, ask for any email and do 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. Yet another reason for ISPs to prefer POP.

Google mail supports both POP and IMAP protocols.

(P.S. I do not mean to sound like I am having a go at ISPs. I think the reasons I've attributed to them above are very pragmatic and sensible.)