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If someone is set to Out of Office and you try to send them an email, before you hit the SEND button, you might get a message that states the Out of Office message. This is a MailTip. There are other MailTips as well. The most common are enabled by default. The advanced MailTips are disabled by default.

One advanced MailTip is a notice when sending an email to outside the company. This can be helpful for certain companies. Here is how to see the current settings:

To get the MailTip setting:
Get-OrganizationConfig |findstr /i mail

To set a MailTip to show when sending outside the company:
set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled $true

Ideally, I wanted to turn this on for a group of people but leave it off for everyone else. I could not find a way, so I just left it on for everyone. Going ISO27001 and getting certified will probably encourage this setting anyway.

NOTES:
https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/mailtips/mailtips
https://docs.microsoft.com/en-us/powershell/module/exchange/organization/set-organizationconfig?view=exchange-ps