Currently the Smart Alerts feature of Outlook Web Add-ins has, Office.MailboxEnums.SendModeOverride, which only supports overriding with PromptUser. This requires the manifest to be configured with SoftBlock for the override to be useful.
It would be more desirable to configure the manifest with PromptUser and then override with SoftBlock. This would be used in situations where the add-in was run and detects a condition that requires corrective action before the message is sent.
The motivation for having PrompUser set in the manifest is to allow users to send emails more easily when they are having network connectivity issues or working in Outlook offline mode.
It's understood that this creates opportunities where a message that would have been blocked is allowed to be sent due to circumstances that prevent the addin from successfully running. This is an acceptable tradeoff.