Unable to send mail using my icloud email address

i cannot send email using my icloud email address. i dont have any issues with receiving mails. i tried using icloud.com on my pc to see whether its working, and i was able to send emails properly. but everytime i try to send something using my ipad, the message gets stored in the outbox continuously loading, they never get sent.

but i use a gmail account too, using the same ipad mail app, but i can send the mails without any issues.