Strange email delay to Outside Recipients

Yesterday one of my users sent an email to 4 recipients at approximately 7:35pm EST

1 recipient was an internal user (on our Microsoft Exchange 365 account) - the rest were outside recipients (a mix of GMAIL and 1 prodigy.net user).

The sender sent an email with a brief message and 4 small .PNG images (average 12kb in size).

The internal recipient received the email within seconds but all of the outside recipients only received the email the next day (this morning) approximately 12 hours later.  

I checked the service health and there were no outages and it seems strange because I know that it wasn't stuck in her Outbox because the internal receiver received it almost immediately and the delay was for external recipients.  Does anyone have any ideas?

Thank you.