Client confirmation emails always treated as Spam by Gmail

Hi

Has anyone had problems with confirmation emails going straight into the client's Gmail Spam folder? This is happening with everyone who makes an appointment with us who has a Gmail address (including my own) - and that is a high % of people.
I don't have particularly spammy text in the confirmation email - in fact it contains a lot of specific details about the appointment. It also uses some of the shortcodes, eg CLIENT, EMAIL. DATE_TIME
Anyone got any tips? It is important that the clients see these confirmations!