Mark addresses outside certain domains. When you are addressing a message to a recipient that's not in your organization's domain, Mail can color the recipient's name red to alert you.
This feature “Mark Addresses” already exists in iOS mail.
1) Open the Settings app on iPhone and then go to “Mail” settings
2) Scroll down and tap on “Mark Addresses”
3) Enter the domain(s) you want to exclude from being marked (for example if you want to flag every email that is not to/from acme.com you would enter “acme.com” as the domain) *
4) Exit out of Settings when complete, any new email that is being sent to, replied, or sent from a domain outside that list will now be marked in bright red to make it obvious it is not in the domain list
Now when you send a new email it will be very obvious if a domain that is chosen which has not been approved in the aforementioned mark address list, as those addresses will be highlighted as red text.