I run a website where people can book appointments. The e-mail address field is optional, and next to this is a checkbox to receive a confirmation e-mail. The confirmation e-mail contains a code to alter the appointment afterwards. This code is also displayed on screen after making the appointment, so people can write it down. Currently, the checkbox is not checked by default but it appears that people often forget to check the box and cannot change their appointment afterwards.
My question is: should the option to receive a confirmation e-mail be checked by default?
The reason why I didn't was to avoid spamming people, although one might argue that when people enter their e-mail address, they can expect to receive an e-mail. Also, the e-mail address can be used after the appointment to send additional information (with consent of the person), so the more people enter their information, the better. When they see the box checked, they might be more reluctant to do so.