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Use the dedicated mention placeholder when your application knows an email address but not a Slack user ID:
The send pipeline resolves the email in the connected Slack workspace and produces Slack mention syntax when the lookup succeeds. An ordinary {{email}} placeholder only inserts text.

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Prerequisites and verification

The Slack installation needs users:read and users:read.email. The email must belong to a user in that workspace. Workspace policies, deactivated accounts, or a different login email can prevent resolution. Send to a test channel and check that Slack displays the intended mention. Do not treat a browser preview as proof of a successful Slack lookup. Avoid relying on an unresolved address as a notification. When the destination itself should be a DM, use recipient_email in the send API; mentioning a person in a channel does not change the message destination. Connect Slack · Placeholders