This might make sense --
I found this buried in the middle at
http://www.ahfx.net/weblog.php?article=107
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Yahoo implemented several MTA changes in Oct. ´06 and one of them was to limit the max. amount of messages per connection to 5. This threshold applies to all senders and there is currently no way around this measure--even for whitelisted senders/IPs. The result of this is that if you have 100 messages and your server is set to retry every 10 minutes then it will take a long time to send and usually timeout, especially under heavy amounts of email. Even if you sent no new mail to yahoo, 100 queued messages would take over 3 hours to send.
Recommendations are:
limit to 5 messages per connection(specifically to yahoo.com, if possible)(if unable, then decrease retry intervals) limit to 50 recipients per message increase of concurrent connections per IP (within reason) Hope this helps everyone!
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I tried manually forcing the queue repeatedly, and found that, in fact, about 5 messages get out of the queue to yahoo on each run.
Anyone else able to confirm that this is really what is happening?
Any info on the server impact of increasing the retry frequency as suggested by duranduran?
Thanks - Bill