Agreed, I have seen the same issues with some mail servers that treat 45x responses as a failure and NEVER retry.

Also, I did a bit of research when I tried out GrayWall and found that some of the spambots, spam relays and proxies have now been updated to retry when they see a 45x error. This means that the spammers and malware authors are starting to adapt once more.

If you want to see my blog posting about my initial results using GrayWall it can be found here: http://momusings.com/momusings/2007/07/experiments-in-spam.html

At the moment I have disabled GrayWall. I may give it another go later this year to see if the bad guys and girls have adapted their creations any further.

Hope this is of interest?

Regards,

Martin

I read somewhere, about 2 years ago that several list server software packages respond to 451 as a delivery failure but react in a more civilized manner to 452. Perhaps having the option to pick the error code that GreyWall issues would be a good idea. 

Technically speaking any 45x code is a temporary condition and a re-send should done. Some programmers should read the RFCs and understand them.

If any one is not using GreyWall I would highly recommend it. This will do more to keep out spam and viruses with the least performance hit. Then clamwall and spamhalter will have less work to do and reduce the overall performance hit of keeping the mail clean.

GrayWall rejecting connection by reply to various SMTP commands depending on situation.

452 error code is little problematic, because it is not allowed as result for all needed SMTP commands by RFC specification.

However, as I reading specification now, maybe will be better to use error 421 instead.

The whitelists for Greylisting at puremagic.com (and greylisting.org) have mentioned the problem some servers have with 451 responses.  But there has not been a suggestion to use another.