change sender address when sending mail through gmail in c#

This is the solution:

1. use the codes above to set mail.From = new MailAddress(address, display name)
2. in Gmail, go to Mail Settings >> Accounts and Import.
3. Add the email account you will use as sender in "Send Mail As". (tick as Alias)

This works for me

Gmail doesn't allow you to change the FROM to something different than your gmail account.

It doesn't matter what you use, they over-write it, before they relay it on. This prevent spamming/spoofing.

Yes just use the From property of the MailMessage

eg.

mail.From = "[email protected]";

EDIT: Also, see this post for more detailed info on how to emails via gmail in C#

Sending email in .NET through Gmail

EDIT: Although this works for mail in general, it appears this won't work for gmail as google overwrite it before its sent (see @Dave wanta's answer)

If you have a limited number of senders you can do as @philip suggested. For instance you may have [email protected], [email protected] and [email protected] or even [email protected]. As long as they are approved senders on the actual gmail.com website you can send from them.

Gmail.com : Sending mail from a different address

If you are expecting to send from an arbitrary user (such as a customer service form on a website where the user enters their email and you don't want them emailing you directly) about the best you can do is this :

        msg.ReplyToList.Add(new System.Net.Mail.MailAddress(email, friendlyName));

If you're in a controlled environment this works great, but please note that I've seen some email clients send to the from address even when reply-to is specified (I don't know which).