There are more reasons than a conspiracy to get you to pay a subscription...
True, but in most or almost all, it is not your sendmail or your server that has anything to do with it. A bounce would indicate a problem or other things...but 'defer' could mean anything. I get a lot from yahoo, both with the stupid ID things and one server without them. They can defer for a while.
I seldom get a defer from anywhere else, but a mailout of a large list could get some.
But I see nothing you can do at your end if you are sending it other than sending a letter to the deferring agent if you have the email, you know who it is. If it is a big company like yahoo or hotmail, I would just send a letter to the recipient and tell them about the issue so they can expect a delay.
Anyone notice how little junk snail mail they are getting now? Or how many snuggie blanket ads they are receiving via email....strange world out there.