capture email domain from email address?

Hey all,

Thought this would be super simple but I am running into some walls. Any help would be appreciated:

Requirement:

• Pull the domain from the contact email address and populate to a field on the contact form.

Background:

• We are a membership based organization.
• Membership is at the Account level and passed down to the contacts connected to the Account.
• Sometimes we get contacts signing up and need to Our Accounts hold membership that pass down to the employees in that Account. We have approved domains and want to quickly identify contacts who have a member email domain. 

Thanks for taking the time to help me resolve this issue. Any thoughts would be helpful. So here goes: