The reason I was asking for information was not to start a C/R flame war. It was because some articles on greylisting talk about how non-compliant MTAs can break the greylisting system by NOT sending back legitimate messages after the delay, or by seeing the delay response as an error and reporting it back to the original sender.
Your first issue of MTA not attempting to redeliver, I wouldn't worry about it much. Such MTA is going to loose tons of other mail too, and whoever was using it, will soon realiase he/she needs something better.
The second issue is more problematic. It can be (for example) result of pure misconfiguration. For example, somebody sets queue runs to go every 5 hours (instead of intended 5 minutes), and with sendmail's default timeout for sending warnings, the sender will get warning message when he sends to greylisted address. I wouldn't worry much about that either, reasons being:
It is very rare.
Even if user gets the warning, it will clearly state it was warning, and that he/she doesn't need to resend the message (at least that is how sendmail's warning messages look like).
The mail will be delivered at the end.
Site that has such an (mis)configuration, would soon realise mistake even if nobody in the world used greylisting. There would be tons of warning messages for non-greylisted addresses too. I don't have that many users around here, but outgoing mail queues on my mail servers are all but empty (for reasons not related with greylisting). If I had such an buggy MTA, my users would be annoyed with it even for sites that do not implement greylisting.
Most of email communication tends to be on regular basis. If your greylist filter whitelists addresses for long enough period of time (one week, or even one month), only the first mail between particular sender and receiver will be greylisted (and generate spurious warning message), and all other communication will go uninterupted forever. For example, as soon as you receive first mail from this mailing list, all other mails from this mailing list will not be delayed (unless you manually delete your greylisting database, or mailing list server changes its IP address, for example).
-- Aleksandar Milivojevic <amilivojevic@xxxxxx> Pollard Banknote Limited Systems Administrator 1499 Buffalo Place Tel: (204) 474-2323 ext 276 Winnipeg, MB R3T 1L7
