Greylisting is a feature that Yahoo has started to employ to protect itself against spammers. However, sometimes it affects regular users. More information about Greylisting can be found here: http://en.wikipedia.org/wiki/Greylisting
What happens is that when you send more than XY (an amount determined by Yahoo), it places you in the Greylist. Once you're located in the Greylist, the email messages will not be delivered or they will be delayed for up to 48 hours. After a period of time, you will again be automatically released and be able to send to Yahoo without any issues.
Below, you can do the same test to see if your IP is GreyListed:
# telnet 66.196.97.250 25
If you are returned a message something like:
"421 Message from XXXXXX temporarily deferred – 4.16.50. Please refer to ….."
This means that you are GreyListed for a period of time. To avoid such issues, please try to AVOID forwarding messages to any email host (e.g. Yahoo, Gmail, Hotmail, etc…) and use the POP3 / IMAP / Webmail functionality that comes by default with your hosting account.
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