That's OK. Many businesses don't have email addresses for their customers. This is a great time to start collecting them. GatherUp works great on mobile devices like iPhone, Android phones, iPads, etc. so you can just add the customer's name and email address right there while they are in your office.
Some companies just use a clipboard, and ask their customers to add their name and email address after their visit. Ultimately this depends on the type of business you have, but collecting email addresses from your customers is a best practice for any business, and can lead to better customer relationships and more repeat business.
GatherUp also provides a short direct feedback "URL" unique to your business, which is found under "Account -> Business Detail". The URL can be added to printed media, invoices, bills, tabs, receipts, business cards, QR codes, SMS etc - and does not require you to know the customers email or name - but will capture the clients email during the feedback process.