My application running in google app engine was unable to send emails for more than a day. All api invocations resulted in the connection being immediately closed from the mailjet side. The resulting ticket suggested it could be that the sending IP address was blocked owing to DOS prevention. But google cloud egress uses a shared pool of IP addresses so I have no influence over the ip address. Mailjet should accept the reality of the cloud situation and never block pooled ip addresses of reputable cloud providers. Mailjet is supposed to be a recommended service provider to google cloud customers.