Uptime Monitor Alert - no email received

Hi, as the title suggests....

I have many heartbeat monitors setup with a target email address (to) and two (cc'ed) email addresses.

I am receiving the email on the (to) email address and one of the two cc'ed email addresses. What might be the reason the other email address isnt receiving the mail?

Note, all the email addresses are on our companies domain. Is there an "opt in" that was never actioned on one of the mailboxes?

Thanks,

Jason