It is a great pleasure for me to send you this letter
We have been using your services from past many years now.
I am glad to inform you that we are happy with your product and with your services.
We have found your people to be very innovative, very helpful and very concerned about the clients and their needs.
We have received quick support from your team and have found them very cooperative.
Our business has grown with your innovative ideas and services and we continue to use your services for many ... many more years to come.
We wish you a great success in all your endeavors.