If they wanted to be good email neighbors, they would.

The other evidence, however, suggests that "good email neighbor" is not a goal for either Yahoo or Google.

The expectation among the general population is that email is delivered in minutes. Any deviation from that timeline is a negative user experience. Failing fast and telling the sender is the best approach. Silently failing to deliver an email until 3 days later would cause more issues.

At least some years ago, Gmail used to retry for 2 or 3 days before giving up.

It still does in some cases.

For example of you send a message from Gmail to another Gmail user with a full mailbox, it keeps retrying delivery for many days.

There may be a plethora of differences but the retry delivery has nothing to do with the client you're sending your mail from.

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

you are confused about the difference between MTA and MUA

SMTP relays should generally, and have in the past, retried for several days.