It is possible to compose a birthday email via the desktop. The system then checks daily whether active customers (customers with an active product and/or customers with an upcoming booking) have their birthday and then sends a congratulations to the customer on the birthday around 8 o'clock in the morning.
You set this up via Messages > Follow-up emails > Birthday email.
In the birthday email you can add formatting, just like photos (via a link to an external website) or links to another website.