I suggest you create a case with GTAC.
You can take a trace on the XMC server to see if the email is sent. Other than that we'll need to enable some specific debug on the NAC appliance to see the email request sent.
Sometimes the SMS message contents cause issues with their delivery. There are message keys in the NAC you can try to modify.
https://gtacknowledge.extremenetworks.com/articles/Solution/NAC-Captive-Portal-SMS-Verification-Text...
Not your issue, but a different type of problem resolved by message keys.
First I would see if the email is being sent to the vendor thought a tcpdump. We will send an email to the #@att.com (Or whatever carrier was provided)
Thanks
-Ryan