We've had a similar experience. We use them for both marketing mass emails and for ingesting support ticket emails (I believe that's through Mandrill). Someone sent a marketing email they didn't like (they said another one of their customers complained about it, or something; literally one complaint) and so they silently shut off our incoming email. We noticed it when one of our customers called to ask why we hadn't…
That's what a secondary MX is for.
Aside telling the sender or receiver, there's no good way to know. I guess you could send a test email every hour.... But this is an evil failure mode. This isn't an accident.