The Mailjet API is spread under 3 versions.
The Email API is available under v3 (URL: https://api.mailjet.com/v3/). This includes all REST API resources (URL: https://api.mailjet.com/v3/REST) and one transactional API resource - https://api.mailjet.com/v3/send.
Mailjet offers also a second transactional API under v3.1 (URL: https://api.mailjet.com/v3.1/send), which includes advanced error management.
The SMS API is currently under v4 (URL: https://api.mailjet.com/v4/).
One of the main differences between the v3 and v4 versions is a change in authentication:
The URL paths for every API resource can be found in the API reference under the respective resource name.