We are using send grid as well. If nothing else helps you can try this piece of code to send a mail and look if it works: send_grid.ex · GitHub
Just to exclude the possibility that there is an issue in the lib.
If :ok is being returned, then SendGrid is saying that it’s a valid request. Do you by chance have the sandbox_enabled flag as true in your config? That would be the only way you’d get :ok without the email being sent.