If you use Exchange defender and are experiencing a delay on sending emails to yahoo recepients, here’s the workaround:
Go into Exchange, then into Organization configuration, then to Hub transport. Once there, put in a send connnector. Click on New Send connector, and on the first tab
Then in the second tab make sure it will only respond to SMTP addresses to yahoo.com
Then on tab three ensure it just uses dns
And on the last tab your default server will be listed as the default-first-site-name.