Sending “on behalf of” emails

I have been received a lot of emails "on behalf on". For example, the AddThis plugin sending a email from "addThis.com on behalf of myfriend@gmail.com".

How do I do this in C#/ASP.NET? Also, does this work if we use gmail for our SMTP, albeit branded to our company domain?

I'm also wondering if there are any concerns about this being unprofessional or getting flagged as spam on the client PC? In other words, have you guys actually implemented this...