I have also noticed this at our shop. I believe the cause is yahoo.com has a fairly low threshold for invalid addresses. I don't know what the threshold is but if I look through the mail_logs at the time they start greylisting us I can usually find a number of e-mails that we attempted to send to them at invalid @yahoo.com addresses. Other than getting internal users to maintain their outgoing mail lists I'm not sure there is anything that can be done. I'm interested to see what the rest of the IronPort Nation has to say...
adding to Jasons answer, which is BTW correct, you can also check the logs or message tracking for these soft bounces. Usually Yahoo includes an URL in the soft bounce pointing to a more detailed explanation, whenever the message was delayed to policy reason or because, as Jason stated already, they have low thresholds on how many messages and recipients they allow per hour. In case of the latter, and if your appliances are running AsyncOS 6.5 or later, you can add an entry for yahoo.com in your destination controls (GUI: Mail Policies->Destination Control), and add rules there to limit the amount of message sent at one time. I recall a working setting for yahoo was
Yahoo.com 8 concurrent connections, 15 recipients in 1 minutes
Of course, as we don't know the specifications Yahoo has, this is just a suggestion and maybe will have to be modified. Would be nice if anybody who has this setting could let us know if that still works.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...