You can use this small plugin to force NinjaForms to validate that a supplied email domain if associated with an accredited academic institution.
To use, simply copy this entire directory into your WordPress plugins directory and then activate the plugin from the WordPress admin panel.
Email addresses with the NinjaForms field name "user_meta_email_academic" will be managed by this plug.
This plugin relies on the JetBrains SWOT database. That database can be found under the "fields/domain" directory. You should consider periodically updating that database. The format of the database can be found in the "readme.txt" file located in the "fields" directory.
You can add additional email domains by adding directories and text fields using the same format into the "fields/extra" directory.
Many thanks to JetBrains for supplying the database used (under MIT license terms). That database can be found at https://github.com/JetBrains/swot. Also thank you to the original authors. The original version can be found at https://github.com/leereilly/swot.