This is caused by one of 2 things.
- the “from” email address does not match with any of your WordPress user’s email addresses., or
- the user does not have permissions to publish
You can set the “Notify on Error” setting to get a detailed message about what is going wrong.
For 1 make sure the email address in the WordPress user matches the “from” email address you are sending from.
For 2 make sure the user has the appropriate permissions that match with “Roles That Can Post”