If you want to show your customers how much they mean to you, communicate with them regularly and do things that are meaningful to them. Why are you sending an email when you can pick up the phone and talk? Have you considered sending them a hand-written letter on their birthday instead of a text?
During daily communication, emails regularly get misinterpreted because we read them through the filter of how we’re feeling at that moment, not through the feeling of the sender. However, when you speak to someone, you can hear the enthusiasm and sincerity in their voice.