Fix Missing Emails in Spreadsheet

I have a customer list stored in a set of Excel worksheets that was compiled over time and, as a result, many email cells are empty. I need the file returned to me with those gaps intelligently filled so every record carries a valid, properly formatted email address... (Budget: ₹750 - ₹1250 INR, Jobs: Data Analysis, Data Cleansing, Data Entry, Data Management, Data Processing, Data Scraping, Excel, Visual Basic)