You need to select the AWS region that best suits you according to your subscribers’ location. Remember to choose this exact same region when you request Amazon for an increase of sending limit. If your AWS account is already out of Sandbox (for a specific AWS region), use that region for MailBluster as well.
For example, here you can see that the sending limit has been increased for the “US East (N. Virginia)” region:
In MailBluster, “US East (N. Virginia)” needs to be selected as SES region when setting up AWS credentials like Access key and Secret Access Key.