Here is a video explaining why some emails are suspended - https://constantcontact.
If somebody isn't getting emails it isn't readily apparent when you look at the record. If you click Engagement you can see if they are getting emails:
If the person thinks this is an error you can unsuspend the account. Go to Reporting and scroll down until you find the email they should have received and press the number below Bounces:
Here you will see the list of people who did not receive the email for a multitude of reasons:
If you click on the ... to the right of the name you now unsuspend the email (you can only do this once):
You can also click the Quicksend to try to send the email again.