Is there regulations to be applied when a customer emails a residential 1003 application for a loan?

Yes, all of the same regulations that apply to a regular application plus e-sign.

is there a link or a website i can get on the e-sign requirements to read up on it?

What is the source of the blank form (you or unknown source) and was the completed application always in electronic form (instead of scanned paper).

customer picked up blank paper application at a branch and then turned around and emailed the completed application.

You don't need to comply with ESIGN because you did not e-deliver any documents.

As a security consideration, the customer has placed his/her personal financial information at great risk (unless the 1003 is encrypted.) Hopefully, your policy on acceptance of credit applications does not permit you to handle this as a credit application, AND hopefully none of your employees suggested that this is an acceptable way to submit an application.