...having found your service by chance, we've been completely happy and pretty surprised actually. You offer a lot of features, with a very reasonable & logical rate system. ...How much your service will help is possibly beyond your understanding, but I'm sure you've heard that from other people using your service.
I just wanted to let you know how much I have enjoyed and continue to appreciate your company. It has been a blessing for saving hours upon hours of personally dealing with e-mails and has turned out to be more affordable than I thought.
It has always been a pleasure dealing with you.