To mention someone in a Slack message, use <@USERID> format instead of "@/display name".
You'll need their Slack UserID, so use the "Find Slack user by email" enrichment first to get the UserID, then reference it in your message with the <@USERID> format.
To mention someone in a Slack message using their USERID, use the format <@USERID> in your message.
You can get the USERID by using our "Find Slack user by email" enrichment first, which will return the user's Slack ID that you can then reference in your message.