shoaib
Then what? Should I change my ISP?? (very difficult task if not impossible)!!!
I don't understand your attitude of confusion.
You have been given good advice in replies 6, 9, 10 etc and yet you still seem to not understand what the issue is.
Those ISP's are blocking you because of the mail sending pattern your server does.
The way to resolve the issues is to fill in the form at hotmail (& other ISP's tech support pages) eg
https://support.msn.com/eform.aspx?productKey=edfsmsbl&ct=eformts&scrx=1which will eventually get your server/domain/IP whitelisted at hotmail, then they will acecept lots of emails from you without them being in fear that you are a spammer.
As stated before, you will have to provide many (almost confidential) details to them though, which involve running tests on your server, as per the data being requested in the form fields.