No appointment email notifications

My client is not getting appointment notifcations, but the client's one seems to be going out. Seeing this on the Logs:

[October 11, 2018 15:04] Unable to send client reminder about the appointment ID: 519.
[October 11, 2018 15:34] Unable to send client reminder about the appointment ID: 520.
[October 12, 2018 05:03] Unable to send worker reminder about the appointment ID: 510.
[October 12, 2018 05:33] Unable to send worker reminder about the appointment ID: 511.
[October 12, 2018 06:03] Unable to send worker reminder about the appointment ID: 512.