> On 28 August 2019, at 19:50, Eliza <e...@chinabuckets.com> wrote: > > Hello, > > on 2019/8/29 10:42, Andrew Bernard wrote: >> 421 Temporary System Problem. Try again later. >> 421 Try again later, closing connection. >> 421 Server busy, try again later. >> The SMTP error 421 is normally used for temporary problems on the mail >> server or a problem with the recipients email account. >> Some mail providers might also return 421 after you reached a limit >> (restriction) on your mail account (see SMTP Error 451 below). > > But I know some antispam systems get abused to return 4xx even there is > neither temporary system problem nor server busy. They just doubt this is > suspicious message and return 4xx codes.
Greylisting used to be a very effective approach to spam blocking. When I first implemented it a number of years ago, it blocked about 95% of the incoming mail. I believe the number of false positives was under 10. However, greylisting was based on the high cost of queueing (disk space and internet bandwidth). When the cost of those fell, spammers were able to implement queueing at very little cost. As a result greylisting became fairly ineffective. It was blocking less than 5% of the spam when I discontinued it. I suspect that most of us who implemented greylisting considered the receipt of spam to be a system problem and hence the 400 series codes were appropriate. YMMV. -- Doug
