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.
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.