A regular email message can't do calculations and custom fields. An Outlook custom form can, but it would not be an appropriate solution except inside an organization, and it's not clear that's the scenario you're working with. (For starters, only people using Outlook can see/use Outlook custom forms.)

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> Hi I am trying to make a automated email for holiday bookings at our
> property.
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>
> thanks Tim