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If you select "Set as notification message", you can mark the message as a Subscription confirmation request or a Welcome email, and set it as the list default, if you wish so. Please note that, in order to use it as Subscription confirmation request, a message must contain the subscription link, otherwise this selection will be disabled, as in the example below.
Finally, place the pointer on the preview image and select "Open preview" to have a preview of the message.
In all cases when the message contains personalized content, the platform will allow generating a preview based on email address - typing the email - to apply a filter or randomly pick a recipient in a group.