Mail: There was an error testing your email settings I'm trying to use mailgun to send emails. I'm on my local machine and I get the error: "There was an error testing your email settings." The API key matches up correctly and nothing of significance seems to be showing up in my logs. Can someone suggest another way of debugging this? The error message isn't too helpful on it's own. Thanks.
Check your Craft storage/logs folder for the underlying error message.  You can also use these knowledge base articles to help debug what's going on based on what you find:

https://craftcms.com/knowledge-base/troubleshooting-email-errors

https://craftcms.com/knowledge-base/why-doesnt-craft-send-emails