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Avoid using the wrong email addresses in Mail on Mac
You can set an option to have Mail display email addresses that don’t end in a specified domain (such as “@apple.com”) in red, to help you avoid sending messages to unintended recipients.
In the Mail app on your Mac, choose Mail > Settings, then click Composing.
Select “Mark addresses not ending with,” then enter one or more email domains, separated by commas.
Now when you address messages, email addresses that don’t match the specified addresses or domains appear in red.