Hi Kane,
You might consider using Mail Tips, which would generate warning even before emails are sent to external user. It warns right on filling the TO: CC: BCC: fields.
MailTips:
https://technet.microsoft.com/en-us/library/jj649091(v=exchg.150).aspx
The External Recipients MailTip is displayed if the sender adds a recipient that's external, or adds a distribution group that contains external recipients.
This MailTip informs senders if a message they're composing will leave the organization, helping them make the correct decisions about wording, tone, and content.
Set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled $true
Set-OrganizationConfig:
https://technet.microsoft.com/en-us/library/aa997443(v=exchg.150).aspx