Email shows as Not Valid but i verified it and I cannot add them back to email list

One contact has the red circle with a line through it next to their email saying the email is not valid. I contacted the client and verified the email is correct and yet I still have not been able to change the status and opt the contact back in for email. Please help.

First, I would check with your SMTP provider, it appears you're using Mandrill. This way you can see exactly why it was set to "Not Valid" in the first place. You can do so by logging into Mandrill, going to Settings > Rejection Blacklist and searching for the email address. This will tell you whether they marked you as spam or it was a soft or hard bounce.

Second, you can mark a email as "Valid" at any time simply by resaving the email in AllProWebTools. You can resave it by clicking on the email address in the CRM record and clicking the green check box to save it.