Well I would say that it's a firewall issue, either on your end or the ISP's, that is blocking the connection. It's not a DNS issue as the server was able to figure out that it needs to connect to d.mx.mail.yahoo.com and it has the correct IP address for that.
I know that that server was accepting connections on the 16th at least...
Code:
Mar 16 15:23:21 email postfix/smtp[9102]: A8A3C904002: to=< user@yahoo.com>, relay=d.mx.mail.yahoo.com[209.191.88.254]:25, delay=79, delays=0.16/0/1.2/78, dsn=2.0.0, status=sent (250 ok dirdel)