Normally, the contents of the emails you're talking about are all hosted on the web - the body of the email just contains links to the relevant text and images. Do it that way, and no-one can change the contents of your email.
Depending on which email client you use - can you create Stationery? I know you can in Outlook Express and base it on a webpage. So my way of thinking is to create a webpage on your pc (in Word maybe?) and then base the new Stationery on that.
The Help file says: quote/ "To create stationery for outgoing messages
You can use the Stationery Wizard to create new looks for your e-mail and newsgroup messages.
On the Tools menu, click Options, and then click the Compose tab.
In the Stationery area, click Create New.
To use stationery you see in a message from someone else, select the message, and then on the File menu, click Save as Stationery.
A stationery file is an HTML file and can be edited in Outlook Express. In a new message window, on the View menu, click Source Edit, and then click the Source tab to make changes.
(Check the size of your stationery inside the Stationery folder at \Program Files\Common Files\Microsoft Shared\Stationery.) " / end quote
I've never done the above but it might be what you are looking for.
Yes I know Outlook Express is dated, but that's what's on this pc I'm using tonight :)