recipients receive emails, but I get an undeliverable error like they didn't

Hello and thanks:

I use outlook.com as a browser-based web mailer. 

My outbound emails are still being successfully received by others, but my own outbound servers seem confused into thinking they were not accepted, or something.

Even after people reply to the original email, I eventually receive:

From: postmaster(at)outlook(dot)com

Delivery has failed to these recipients or groups:

(various emails at unrelated providers)

Your message wasn't delivered because the recipient's email provider rejected it.

(end of error)

Why does it think they were rejected when they were accepted? It's not sending duplicates, re-trying, thankfully.

Thanks for any insight.