Hi! I have same case(HTML Mailer not working), but I have a different error. In SendGrid the domain is verified, the sender is also verified, the API-key is generated and added to plugin. But when I try to send a letter, I get this error:
Sorry, we encountered a temporary error and cannot fulfill your request. We will fix this as soon as possible. Please try again later!
The same thing happens when I try to use your API-key and your address as a sender in my application.
Can you help or give any advice?