If you modify the xml file it will change the format for ALL builds. I don't think it is possible to specify a particular build and send a specific message for it.
A better option might be to override a target (perhaps AfterDropBuild) and insert a Mail task that will send out a mail with the content you'd like. Check out the MSBuild Community tasks for the custom Mail task, or you could create your own.
RamyaP
Posted: Team Foundation Server - Build Automation, Customize the Email format for Build Completes Notification
U could add the following mail task to any target on the execution of which you want a mail to be sent. As DQM said, override the AfterDropBuild to send out the email
" SmtpServer="smtpservername" />
Thanks,
Ramya
BJHop
Posted: Team Foundation Server - Build Automation, Customize the Email format for Build Completes Notification