Hello. If you use smtp.gmail.com as the outgoing mail server, it will designate it as "sent on behalf of gmail." You will either need to get a Google Apps account with another domain, or use a different smtp server if you want to remove the gmail notification.
This only occurs when using gmail as the outgoing server when the user does not have a gmail account.
What is the current email address they are using? Just include whatever is after the @ symbol.
Hi Sorry forgot to come back and update as we found the problem with the original SMTP server, it was with the provider and now don't have to use the gmail account.