When sent email to a list of clients via CRM, few client claim didn't receive although the status is shown as 'Sent'.

I believe it may not be the CRM's issue because other clients still able to receive the email.

Could you able to explain what (possible reasons) may cause it? I may need to explain to the recipients that this is not caused by CRM and probably due to their email setting or any other external issue.

Thanks,

James