Send Test Email Error in Campaign's Email step: An error occurred, please try again

If you see this error message, or if the Send Email Trigger Action is Not Firing

Please check if

1. the email/sms content for missing } or overusing {{{

2. the custom values are valid:

    especially the links: List of valid Custom values

3. the email addresses are valid:

    e.g. test@gmail,com with a comma as the dot

4. From field is in the format of an email address:

    e.g. Change ABC company to its email like info@abccompany.com