Validating email addresses through temporary mail message
Well, it really depends on why addressesIn is a string. If it's input from the user, as I suspect, it is a fine method.
If I was to be a harsh critic I would ask why a method named "Validate" returns a string.
I think if you rename the method to like "ScrubEmailAddresses" or "SanitizeEmailAddresses" and envision it in an app that lets a user input email addresses to send an email to, it's kind of nifty. Basically instead of a roll-your-own-validations approach this says "hey, let the framework do it for me!" by taking advantage of the fact that .NET will just throw a FormatException if you try to add an invalid address. It's a fairly decent server-side equivalent to a RegularExpressionValidator using the predefined email regex. I also agree with Mike McCaughan's comment mentioning there is a resource hit when having the framework throw an exception.