In his latest post on the SitePoint blog today, Harry Fuecks explains and demonstrates how to send UTF-8 encoded emails with the help of the eZ components library of tools.
One of the subjects I brushed over last week was how you handle UTF-8 in email, because I don't have a full picture on the best way to solve this. The fundamental problem is summarized nicely on Wikipedia's discussion of MIME.
Part of the problem there is there are different approaches to how you can solve this. There's plenty of gotchas and clearly more than than you get by default with PHP's mail function (that's after you fixed your code for email injection). In other words taking a "not invented here" view is going to leave you with a big workload.
So, Harry suggests uisng the Mail classes from the eZ components libraries to send (explicitly) UTF-8 encoded emails. He also includes a simple example of how to send it, showing a message in German to be sent correctly.