This is a new feature of yahoo mail. It is using what is called Grey Listing to fight spam. For more details, you can read: http://en.wikipedia.org/wiki/Greylisting
happens is that whenever a new message is sent to yahoo, it replies
with "try again later" message. e.g. here is a test smtp connection to
one of the yahoo's mx:
# telnet 22.214.171.124 25
Connected to 126.96.36.199.
Escape character is '^]'.
Message from (a.b.c.d) temporarily deferred - 4.16.50. Please refer to
Connection closed by foreign host.
as you can see, yahoo did not even give us (the ip of a.b.c.d) a chance
to start the smtp session. What it will do is wait for us to try again
later. Our mail servers (or yours on your vps/dedicated server) will try
again later and if yahoo likes the delay long enough, it will let the
server communicate further and deliver the email.
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