User not local; please try a different path. The server response was: Sender address is not valid for your login. Check your email program settings.
When you get the above error in plesk please make sure that that in your code for example its set from :
myMail.From = new MailAddress(txtEmailAddress.Text);
to actually show it its sending from the authenticated email address this is how it must look:
myMail.From = new MailAddress("email@example.com");