In sending business greeting cards to your clients, you need to consider three things - your reason, the design elements of the greeting card, and your message.
Bold color palettes, preferably with designs and colors that are the same as your branding adds a personalized touch to your business greeting cards. It just shows that the greeting card is personalized. Try to make your greeting card stand out from the other greeting cards that they will surely receive from their other clients or partners too.
The next challenge would be creating the message for your card. To be able to create a more meaningful greeting card it would be best to write it yourself. You are the one working with a client, the one who understands your history with them, both the good and the bad. Your message will be more sincere when it is created with these in mind rather than giving the task to someone who has no working relationship with your client.
Only say a message that you believe in and create a custom sentiment. It can be the gratitude that you would like to express after such a client provided you with such an opportunity or after giving you a chance to work again with them. Make your message more genuine.