I'm sending emails using the marketing cloud from Salesforce. The send email for the email sent is showing the message "The email used for this send is not valid. Please make any necessary corrections in Marketing Cloud."
What does the "email" in the above email mean? All of the emails used in the "To" and "From" are all genuine. Why is this error message coming?
You will need to Validate the email. Open the email in Email Studio and select the validate button: 
Then choose the Data Extension that you are using for your Event Source in your Event used by the Interaction and you will see any associated validation errors with your email:
The Email that it is referring to there is the email object that contains the content of the email, not an email address. The email content could be seen as not valid if it is missing required pieces such as a physical mailing address for CAN-SPAM compliance or an unsubscribe link if it is a marketing send.
