You have a few options to choose from:
1. Export all customers to a CSV/Excel file, and extract their email addresses from the generated file.
2. Export all you customers to your MailChimp account, and email from there.
Note. If you do not have a mail chimp account, you can sign up for one free.
3. Export all you customers to your Constant Contact, and email from there.
Head to your customer list, filter status to 'All Customers' and then select to export the filtered list for all 3 options mentioned above.