If you activate the email service in global settings and provide your customers email address, then both an email and physical letter will be sent. This makes it harder for your customer to ignore the reminders which achieves better results. It makes sense to always ask for email addresses when an account is opened and be sure to regularly update these.  We find it best to get the MD or owners email address as a generic address such as info@, sales@ or accounts@ might not get seen by the correct decision maker. We quite often come across instances where senior management were unaware invoices were not being paid and an email from us is an excellent way of bringing it to their attention!