Сервер IIS6 SMTP, повторяющий слишком быстро (после 451 greylist)
Согласно passdb документации относительно Samba.org, поле признака учетной записи может содержать до 16 символов. В настоящее время только 11 используются.
Я могу подтвердить, что мы использовали его в производстве много лет с учетными записями, имеющими всего один флаг.
I may be giving a answer you don't want, but IIS SMTP is very simple and limited... you may be hitting a bug. I would smarthost all emails to a more complete SMTP server, like postfix or qmail (both linux based), where they are queued to deliver to the internet. The IIS SMTP would be used as a simple localhost relay, bypassing most of it problems.
As the your problem... are you sure that they are the same email? maybe there are several emails outgoing to the same email and generating that apparent bad retry pattern.
Either way, that 300s in the logs is just a user level comment, the SMTP will not read it, nor try to obey to it. Also, the receiving end have no influence whatever on the retry time of the sender, only the response code (451 in this case) can trigger different reactions on the sender side... but that reaction would be consistent always.