If you are getting 'Invalid Contact' or 'Invalid Lead' error in the MassMailer one-off email feature while sending the email, it is because one or more of the following fields is checked to true in Contact or Lead.

Contact Object

Invalid Contact

Email Opt Out

Lead Object 

Invalid Lead

Email Opt Out

That means the email is invalid (hard bounced, spam reported) or opted out from the email (unsubscribe). MassMailer does not let you send emails to such records. 

If you want to resubscribe any of the leads or contacts then you can following the instructions below.

Step 1: Uncheck the Email Opt Out field to true from the contact or lead details page and also remove that email from Global Unsubscribes in SendGrid.

Step 2:

Log into SendGrid with the account details that were provided to you.

Navigate to Suppressions -> Global Unsubscribes -> Search for the email id -> Select the checkbox and remove the selected record as shown in the following picture.