I recently started getting some soft bounces coming back from Yahoo.
I believe that this is a “greylisting” operation on their part: “Message delivery has been delayed.”
The typical “lists” are Blacklists (all email is rejected) and Whitelists (all email is accepted).
Greylists utilize what is called a soft error, or soft bounce on the email server.
When sending an email, there are some errors that cause the transmission to fail and halt, and there are others that indicate that the sender should “try again later.”
For example, if you try to send an email to an account that doesn’t exist, the sending server will not try to deliver the message again. But if the error is something like “Mailbox is full”, then the sender will keep trying N number of times.
Almost all spammers will not bother to resend these soft bounces. They are kicking of millions of emails, it isn’t worth their time to deal with these soft bounces.
So greylisting does the soft bounce, and sees if the sender tries again. If they do, then yahoo labels them as an OK sender, and allows their email through. If they don’t try again, yahoo can label them a spammer, and block them out.