Emails are not sending for all the recipient

Dear Team I am working on ERPNEXT Version 13.5 in that we enabled notifications but its getting queued in the email queue and the status is Not Sent but while opening that particular email queue i got to know that out of 10 recipients 8 of the recipients email status as sent and rest of 2 are not sent …

Please guide me in that so i can able to solve that.

Thanks in advance,