To download your customer database, go to reports, expand the list of reports, and choose customer export.
You can filter the list using various different parameters, for example, you may wish to only download a list of customers who dined in December or you may want a list of customers you have tagged with a particular quick info label.
When using your customer database to obtain email contacts for marketing purposes, to keep within GDPR laws you new to select to only show customers who have selected to "opt in to marketing messages via email".
After clicking search, you will have the option to export the list as a .xls file.
You could then use this file to upload to a mail marketing website, such as MailChimp or ActiveCampaign, to create a targeted marketing campaign or newsletter.