I see an email back saying delivery failed from the recipient's ISP. Is it safe to assume that the other recipients still got the email? Or is there some kind of signaling back to my ISP preventing delivery to any recipients and thus canceling the email.
No, when you send an email it is sent to all of the recipients individually.
You should get a "delivery failed" for all of the recipients that failed to get the email for whatever reason. If you do not get a delivery failed notification then you should be able to assume that the email got through.
Depending on the email systems in question though it may be a couple of days before you get the notification as servers may try to resend the email and thus delay the "failed" response.