1982 (see RFC 821), but still there are MTAs that get it wrong, usually by bouncing to the address in the From: header. Systems that implement greylisting work fine with VERP if the envelope sender follows the above-mentioned format. However, some VERP implementations use message number or random key as part of VERP, which causes each post to the mailing list to be delayed unless the greylisting system treats "similar" sender addresses as being equivalent. See also Bounce Address Tag Validation (BATV) – for bounces from backscatter Sender Rewriting Scheme (SRS) – for bounces from e-mail forwarding and SPF Simple Mail Transfer Protocol (SMTP) draft SMTP extension to streamline VERP References Email