When you create a service reminder you will see a dropdown with options of who you want to send the reminder to. In these options, you will also see a method of communication - a phone call, email, letter, or an SMS message.
You can change this or add to this list.
In the 'Send to' section, you will see a list of contacts related to this customer. The method of reminder will show, i.e letter, phone, or email. If the method of reminder you want is not in this list it is because it has not been ticked within contact details.
Go back to the customer and open the 'Contacts' tab.
From here, click 'Edit' next to the contact you want to send the reminders to. You will see a page where you can edit the details of the contact. Make sure you have an email address saved, a telephone number, as well as a full address.
There is a section called 'Communication preferences' tick the boxes next to the method of communication you want to send via.
Then you can go back to 'Service reminders' and add/edit a service reminder with the new communication preferences.