There's a big difference between desktop mail clients that have an outbox and web-based experiences of services like Yahoo and Gmail. I doubt any of the latter kind will retry sending an email over several days.
The other evidence, however, suggests that "good email neighbor" is not a goal for either Yahoo or Google.
For example of you send a message from Gmail to another Gmail user with a full mailbox, it keeps retrying delivery for many days.
That is definitely in the realm of the server being tasked with delivering your mail.
In traditional desktop Mail clients the Outbox is a local folder and stores Mail that hasn't been delivered to the Server tasked with sending out the mail.
The retry method mentioned here is about a Mail that has been delivered to the outgoing mailserver which in turn may have trouble reaching the destination server