We are also not all of our email. 

We are still in transition to using Zoho so we receive email on our own server then forward it to Zoho's server. One more technical user noticed that he has a couple of email messges that he received on our email, server, but never got it in his zoho account. Our server log says 

lost connection with mx.zoho.com[204.141.43.44] while receiving the initial server greeting

Is that graylisting? 40 seconds later the server says the email was sent:

Jun 24 11:18:34 (private info redacted) relay=mx2.zoho.com[136.143.183.44]:25, delay=87, delays=0.01/0/46/40, dsn=2.0.0, status=sent (250 Message received)

Though mx2.zoho.com says 250 received the user never received it.