Office 365 / Exchange has the option to warn you if you are sending to external people. To stop that “oops – that wasnt meant to go outside” feeling after you have sent it.
https://social.technet.microsoft.com/Forums/en-US/800c1b34-52d8-44d5-a5e1-186305caf91f/how-to-set-specific-external-domainrecipient-warning-before-sending-by-mailtips?forum=exchangesvradminlegacy
In summary this is how to set that on.
#turn on the mail tip for external emails Set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled $true # to turn it off put $false
