Ride Master makes contacting riders simple and straightforward. In the rider management screen, perform a query that returns the riders you want to contact. For instance, all riders who haven't made a payment. You'll notice two links that allow you to contact your riders:

Email displayed riders using Mailer
This link allows you to use Mailer to contact your riders. Mailer is a separate program that knows how to mass email a bunch of riders. A report is produced detailing who was emailed, the send status of the message and if anyone has opted out of receiving emails from Mailer. You may want to contact the opted out riders directly.

Email displayed riders using your own email client
This link allows you to use your own email client to send the message. Clicking it should start a new message with all of the riders in the to-list.