Way back when - there was a "best practice" by some and talk of a requirement of putting "ADV" in a subject line to denote "advertisement" but I don't believe that ever got a lot of traction. A "do not email" like the "do not call list" is the same.
Email spam filters are also better than they were "way back when".
I would recommend keeping some good records for fair lending and CRA of your solicitations, ie. who, where, any prescreening techniques employed, etc.
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell