Support for migrating from the legacy Mail API to an SMTP-based email service, such as SendGrid, Mailgun, or Mailjet

## Feature Feature Support for [migrating from the legacy Mail API to an SMTP-based email service](https://cloud.google.com/appengine/migration-center/standard/python/mail-to-smtp), such as SendGrid, Mailgun, or Mailjet is in [General Availability](https://cloud.google.com/products/#product-launch-stages).