Yahoo! has a strange delivery problem lately (last week?) for yahoo.gr recepients..
$ host -t mx yahoo.gr yahoo.gr mail is handled by 10 mx1.mail.eu.yahoo.com. yahoo.gr mail is handled by 10 mx2.mail.eu.yahoo.com.
While mx2.mail.eu.yahoo.com accepts mail as expected …
$ telnet mx2.mail.eu.yahoo.com 25 Trying 77.238.177.142... Connected to mx2.mail.eu.yahoo.com (77.238.177.142). Escape character is '^]'. 220 mta101.mail.ird.yahoo.com ESMTP YSmtp service ready helo banana.irc.gr 250 mta101.mail.ird.yahoo.com mail from:<postmaster@irc.gr> 250 sender <postmaster@irc.gr> ok rcpt to:<postmaster@yahoo.gr> 250 recipient <postmaster@yahoo.gr> ok quit 221 mta101.mail.ird.yahoo.com
the other mx, mx1.mail.eu.yahoo.com doesn’t ..
$ telnet mx1.mail.eu.yahoo.com 25 Trying 217.12.11.64... Connected to mx2.mail.ukl.yahoo.com (217.12.11.64). Escape character is '^]'. 220 mta161.mail.ukl.yahoo.com ESMTP YSmtp service ready helo banana.irc.gr 250 mta161.mail.ukl.yahoo.com mail from:<postmaster@irc.gr> 250 sender <postmaster@irc.gr> ok rcpt to:<postmaster@yahoo.gr> 550 relaying denied for <postmaster@yahoo.gr> quit 221 mta161.mail.ukl.yahoo.com Connection closed by foreign host.
Reported this using their Issues Form as a possible misconfiguration on their side, but no answer yet..
Anyone?
Don't fear the penguin 
Here our Yahoo! Mail queues are empty. However, while mx2 seems to be accepting normally, mx1 reseted a lot of connections which later in the day it accepted.
You can never know with Yahoo! Mail and their Postmaster blogs deal only with UI issues.
Got a canned reply earlier from their support of no real value (listed a few FAQs, but not really dealt with the reported problem)..
For the time being, I use mailertable to always route yahoo.gr mails to mx2 only ..
«Got a canned reply earlier from their support of no real value»
The one that says to contact your Postmaster, right? I angrily replied once in such a reply saying that «I am the Postmaster and I am trying to find out what the hell is going on». I managed to get a real postmaster to reply back to me. It may work for you too.
Didn’t get a useful reply the second time either, but the problem went away this morning.. Both mx1.mail.eu and mx2.mail.eu now accept all mails..
Αν συμβεί κάτι τέτοιο, αν θες κάνε μου ένα CC για να πρήξω τον σωστό μάστορα. Στο ίδιο κτήριο με εμένα είναι, λογικά κάτι θα γίνει.
Χάρης.
Ωραία γιατί την προηγούμενη φορά εμένα μου πήρε 8 μήνες να λύσω το θέμα…