Greetings. Based on my tests and knowledge, when you enable impersonation safety tip in anti phishing policy, when a message that fails impersonation checks, a safety tip will notify recipients when the first time they get a message from the sender or if they don't often get messages from the sender, which is used warn the recipient about potentially harmful messages.
Moreover, user impersonation protection does not work if the sender and recipient have previously communicated via email. If the sender and recipient have never communicated via email, the message will be identified as an impersonation attempt.
But since the impersonation check is done at the email filter level and not rendered at the mail client level. Therefore, no matter whether the recipients add the sender as a contact or add the sender to the safe sender list, when a message that fails impersonation checks, the recipient will always see a safety tips of the message.
Given this result, when a sender is legitimate, to prevent the recipient from continuing to receive safety tips, it is recommended that you can modify the anti-phishing policy to add the sender as a trusted sender.