Modern businesspeople often need to send out a similarly-structured messages to a large number of recipients over time (order confirmations, customer requests etc.). One good solution is a predefined template which you can quickly insert into your message.
You can use «Message templates» feature — it allows you to create one or more templates which you can later use while composing new mail. You can enable and configure this feature in your control panel.
To use a previously created template, click in the compose form and select the template you'd like to use. Message body will be replaced by the template (you can revert the changes using the same icon).