I have sent a mail to an address with a suffix of @rediffmail. But instead i wrote @Rediffmail. As you can see the 'R' is capitalized when it is not supposed to be. What I want to ask is that will the email still reach the recipient? I tried testing it from gmail but it automatically changes the 'R' to 'r'.
The domain part of an email address is not meant to be case sensitive so, yes, it should work.
RFC952 was the first standard to address host names, and it states in the assumptions section:
No distinction is made between upper and lower case.
Some of the rules of that RFC have been relaxed by later RFCs but the case insensitivity still stands.