If any of the recipient email is not getting the emails from MassMailer that means that either that recipient has unsubscribed or the email is invalid/spam/bounced. 

MassMailer does not let you send emails to such recipients. 

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

Step 1: Uncheck the Email Opt Out field and/or Invalid Contact/Invalid Lead field from the contact or lead details page and also remove that email from Suppression List in SendGrid.

Step 2:

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

Navigate to Suppressions -> Any of the suppression List such as Global Unsubscribes/Bounces/Group Unsubscribes/Spam Reports/Blocks/Invalid-> Search for the email id -> Select the checkbox and remove the selected record as shown in the following picture.