This post describes how to send email from a Google Cloud virtual machine instance using the Paubox Email API.
According to their documentation, Google Compute Engine does not allow outbound connections on ports 25, 465, and 587. Although standard email ports are blocked by Google Cloud, you can choose a non-standard port to send email through with the Paubox Email API.
Here is a quick reference to Paubox-specific SMTP settings that are used to configure clients in Google Cloud: