We can't show MailTips right now warning in Outlook in Microsoft 365

Original KB number:   3083843

Symptoms

In Outlook in Microsoft 365, you receive a We can't show MailTips right now warning after you change the From address to the address of a non-mailbox-enabled recipient.

You may also see a similar warning when you forward a meeting invite for a non-mailbox-enabled recipient.

However, in Outlook Web App (OWA), you don't see this warning. Instead, OWA displays a MailTip from the current forest.

Cause

This behavior is by design.

More information

The attempt to complete a MailTips lookup fails if the From address that you specify isn't associated with a mailbox in the current user's forest when you use Outlook. The MailTips feature tries to complete the lookup under the security context of the user in the From address. The attempt fails if the recipient isn't a mailbox user in the current forest.

For example, if the address is associated with a distribution group or a mail user, you receive the following error message in the Outlook logs:

The ActingAs parameter does not match a user in the directory.

MailTips from the current forest are displayed in OWA.