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

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 66.196.97.250 25
Trying 66.196.97.250...
Connected to 66.196.97.250.
Escape character is '^]'.
421 Message from (a.b.c.d) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html
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.

To avoid such delays and problems with yahoo mail, you are encouraged NOT TO FORWARD emails to yahoo or any other ISP. Please check below

If you are on one of our shared /  reseller server, then this usually means that other users or you have setup email forwarding to AOL and causing a block at AOL. This is because when an email arrives at your domain, and if you have setup email forwarding to any of the following ISPs email addresses:

aol.com
att.net
bigpond.net.au
comcast.net
mac.com
videotron.ca
worldnet.att.net

the email will be forwarded to the ISP email address as it is, which can include spam as well.

What happens next is that either the ISP's mail filters or the recipient marks that email as spam, which results in your hosting server being considered as source of that spam.

Here is an example of this issue posted on AOL:

http://postmaster.info.aol.com/errors/421rlynw.html

[ When an AOL member clicks "this is spam" for a piece of email sent from one of your IPs, this is considered a "complaint". ]

This makes you the spammer and your server gets blocked at any of those ISPs. All these ISPs are known to use incoming filters and do not work well with email forwarding.

Solution:

1. If you have a dedicated server or vps, all users on your server should remove any forwarding to such ISPs and create proper email accounts (pop3/imap/webmail).

2. If you are victim of the above problem and are using one of our shared hosting account, you can request support to get your account moved to a different server that is not being blocked by AOL or wait for AOL to unblock your current server, which happens automatically.

3. If you are using a forwarder like this, please remove it.

4. If you are trying to send email to yourself at AOL, we recommend that you follow solution #1 and create proper email account from your control panel.