I ran into a problem that seems to have some of the same attributes. In
my case Google was rejecting my email, however they may have been a
little more polite about doing so.
Have you checked your DNS and reverse DNS entries. Is your server a
dedicated system with a single IP address.
In my case I was running a multi-use server which had multiple IP
addresses. This resulted in email being sent out on a IP address whose
reverse DNS did not resolve to the published server. Google rejected my
email, but they did issue an error message.
I wonder if Yahoo is doing some sort of DNS to IP check and finding a
mismatch just dropping the connection instead of issuing a valid error
code. Not very polite of them, but past experience with not particularly
unusual for them.
To solve my problem, the first thing I did was to ensure that the rDNS
for my sever was correct.
Next added smtp_bind_address to the smtp entry in my master.cf.
the effect of this is to ensure that the address and reverse address
match, and that any DNS look up will not result in an error.
I don't know if this will solve you problem. but I hope it helps.
