v3.1.0 is a non-critical feature release. The main change is first-class support for AWS SES. If you are sending through SES then you do not need any plugins besides the aws-sdk module to use Nodemailer.
The built-in SES support is an improvement over the previous nodemailer-ses-transport plugin. You can instantiate the aws-sdk object as you wish yourself instead of letting Nodemailer handle the setup. Rate limiting is vastly improved, Nodemailer guarantees the most optimal sending speed, without hitting the actual limits. There’s also an option to specify SES related mail options like Tags or ConfigurationSetName. If you use DKIM signing then Message-ID and Date fields are automatically suppressed from the signature, no special config needed.
You can find all the details required to send mail with SES from the Nodemailer homepage.