Yahoo and Postfix 26
If you’re getting “temporarily deferred” Postfix log entries when trying to send mail to Yahoo! that look like this: host mta7.am0.yahoodns.net[] refused to talk to me: 421 4.7.0 [GL01] Message from (xxx.xx.xx.xxx) temporarily deferred - 4.16.50. Please refer to http://postmaster.yahoo.com/errors/postmaster-21.html or status=deferred (host mta7.am0.yahoodns.net[] said: 451 mta1336.mail.bf1.yahoo.com Resources temporarily unavailable. […]

How-To: Postfix configuration to reduce Yahoo temporary deferrals using Transport ...