That's one of the best-practices recommendations the Big Four -- AOL, Yahoo!, Microsoft and EarthLink -- issued today in a report outlining their best ways to fight spam.

None of the recommendations breaks any new ground for legitimate, ethical bulk emailers, as you can see from this list:

* Do not harvest e-mail addresses through SMTP or other means (defined as collecting e-mail addresses, usually by automated means) without the owners' affirmative consent.
* Register your e-mail domain with a creditable safelist provider.
* Always provide clear instructions to customers about how to unsubscribe or opt-out of receiving e-mail. Promptly respond to these requests.
* Do not use or send e-mail that contains invalid or forged headers.
* Do not use or send e-mail that contains invalid or nonexistent domain names in the From or Reply-To headers.
* Do not employ any technique to hide or obscure any information that identifies the true origin or the transmission path of bulk e-mail.
* Do not use a third party's Internet domain name or allow mail to be relayed from or through a third party's equipment without permission.
* Do not send e-mail that contains false or misleading information in the subject line or in its content.
* Monitor SMTP responses from recipients' mail servers. Promptly remove all e-mail addresses for which the receiving mail server responds with a 55x SMTP error code (e.g., "user doesn't exist").

Each site has a copy of the report and the press release. For brevity, here's the Yahoo! report. We'll post the rest shortly.