An email that didn't send/only sent to few on the group list

I sent an email to our entire organization on March 16th at 3:30pm.  The stats suggest that it was successfully sent to the entire list.  Only a few people received it. The only reason we know is that a married couple reported spouse had received the email while the other had not. There was a 2nd email sent at 4pm that everyone seems to have received.   I did a random poll of people on our list and some reported having received both while many reported having received only the 2nd one.   I don't think this has ever happened before and I hope you can look into it to make sure it never happens again.  It was quite a crucial communication.  I ended up resending the first to the original list at 6pm -- I believe more received that.  Please let me know what you discover and let me know if you need additional information --- Julie Campbell