Looks like phabricator/externals/amazon-ses/ses.php hard codes the host for the service as email.us-east-1.amazonaws.com. Instead, there should be a config setting similar to S3 that allows administrators to specify the host (or more likely the region).
The reason this is necessary is because SES allows different configurations per region, including verified domains and email addresses, and a situation may arise (as it did to me) where outgoing email on the domain fails to send respite the entire domain being validated - all you need is to validate the domain for a region other than us-east-1.