How can I send a mail message that the recipient can't modify?
After a message leaves your Outbox, you have little control over what happens to it. However, Outlook lets you set the Sensitivity property of an outgoing message to Private. When a user in your Exchange organization receives a Private message, that person won't be able to edit the message.
Another approach is to use a digital signature on your message. Signing the message lets the user check not just whether your signature was valid (i.e., that you are who you say you are) but that the message wasn't altered after you signed it. Using a digital signature won't necessarily prevent the user from changing the message, but the signature will show that the message isn't what you sent. Yet another method is to send the core of the message as an attachment. For example, if you compose a Microsoft Word document, you can use Tools, Protect Document to password-protect the document against changes before you send it as an attachment.
Several secure email services have sprung up to meet the demand for verification of message content and delivery and to control what happens to a message after you send it. Some of these services cause a message to self-destruct or expire after the recipient reads it or after a certain amount of time passes. Check out these products: Global Market's 1on1Mail (http://www.1on1mail.com), CertifiedMail (https://ssl.certifiedmail.com), Disappearing Email (http://www.disappearing.com), mailedSAFE (http://www.mailedsafe.com/summary.htm), Sigaba's SigabaSecure (http://www.sigaba.com), and ZixIt's ZixMail (http://www.zixmail.com). Most companies provide a free Outlook add-in to integrate directly with their services.