I believe the problem is that you uses is an "ses-smtp-user.some_number". You see, with SES, there are two ways to send emails.
- using web service (AmazonSimpleEmailServiceClient)
- using SMTP
The user that was created was for smtp, ses-smtp-user.some_number. You are using that credential for a web service call.
My suggestion is to create a normal user (not from the SES screen), and assign SES permissions. Do note that the newly created credentials won't work with SMTP now.
If you want a credential that works with both, SMTP and web service, you may want to read up on this: http://docs.aws.amazon.com/ses/latest/DeveloperGuide/smtp-credentials.html#smtp-credentials-convert