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:

What 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 25
Connected to
Escape character is '^]'.
421 Message from (a.b.c.d) temporarily deferred - 4.16.50. Please refer to
Connection closed by foreign host.

Now 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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