If you are getting into the spam business, you need to consult an attorney. Prosecutors gain great popularity in prosecuting spammers. Everyone applauds, because everyone hates spam. If you spoof the domain names, it looks suspicious. I think you should consult with a networking engineer and an attorney. I have a colleague, a retired attorney, who makes a good living in suing spammers under the canned spam act.
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The CANN-SPAM Act governs this activity. What you're doing is pretty specific, so you will need to consult with an attorney. You can gain some information doing a general search of the CANN-SPAM Act Summary. Therein you may find your answer too. Good Luck!!
The recommendations in this answer are not considered legal advice for the purposes of ethical, legal and practical evaluation, nor does this recommendation create a retention of counsel agreement between us, wherein an attorney-client relationship or privilege exists. These recommendations should never be relied upon without first consulting an attorney in your jurisdiction. I am not your attorney, unless we enter into a written agreement fulfilling the terms of that agreement. The comments posted herein are purely for public educational purposes and public discourse on general legal topics. I presume the specific fact scenarios are only supposition and am thus commenting for the public's benefit and understanding of law. I am not soliciting you to hire me or my law firm, though in certain jurisdictions I may be able to refer you to an attorney that can help you or may consult with you for free.
Yes, from your facts, it appears that you would be governed by the CANN-SPAM Act. Consult an attorney to understand all the ramifications of your proposed actions.
I am not your attorney. Contact an attorney for advice immediately.