Why can't I reply emails but send new one on my iPad 2? I cannot reply to emails on my iPad 2, but if I compose a new one it will send it fine. This is a new problem as it was working fine before and there have been no changes. 

I use a Telstra Micro SIM and an optus email address.
I believe that what is happening is that you have multiple email accounts setup, and when you go to compose you are using the outgoing of account A, and when you reply/forward you are attempting to use the outgoing of account B.

So to fix, check your outgoing account settings for the account that you cannot reply emails for.