SES AWS Error Code: SignatureDoesNotMatch, Status Code: 403
This looks like a clock skew issue. The time of the machine you are sending the request from is too different from the Amazon SES server's time. This is causing your signature to be invalid. Your credentials are probably fine. Correct your machine's clock or sync up with an NTP server.
Along with this solution I have done the following.
1) I have created the user with PowerUser policy in IAM.2) Updated the secret/access key in my code.
Now it's working.
Not sure why SMTP credentials(Which are created under SES Dashboard) are not working.