Yes and no. We know that your other ISP does this, but we take some exception to it. Let me tell you what we do, which should be OK for reasonable people with typical situations. If you are unreasonable or have an atypical situation, you probably should sign up for a dedicated host with us so you can have total control of your email appearance. Learn about decicated server options at http://www.jtan.com/dedicated
If you have a ProShell/DNS or ProWeb account, and you send mail from or through our SMTP relay (non local mail), we will automatically rewrite your From: header to reflect your domain. You can opt out of this feature if you want, but the default is to do this. We will also rewrite some other headers like the "From " envelope and such. So an ordinary MS Outlook recipient of your mail will see nothing but "From: firstname.lastname@example.org". That's what you want, right?
However, our mail server has a name in the jtan.com domain and it rightfully adds it's name to the mail in a Received header, indicating that it handled the mail. Also, if you originate mail on a jtan machine and don't configure your mailer to add certain headers (like Message-ID), our server will add the header using its jtan.com name. None of this "ISP domain leakage" should bug you -- it dosen't reflect any unprofessionalism. Rather, it shows that you do business with an ISP that <em>is</em> professional, has a good reputation, and is a stickler for configuring mail systems to provide proper audit trails that allow effective investigations in cases of abuse.