WhatsUp Gold should have a built-in SMTP service.  This would make it a stand-alone product not relying on any external services for it's operations.

Alternatively, WhatsUp Gold could have the ability to define multiple SMTP relays for outbound notifications.

The problem now is that you can't effectively monitor the mail server that is configured for sending outbound email alerts.  If the server that is down is the SMTP server configured for outbound SMTP messages, then the notifications can't be sent.  Redundancy in notification routing could be achieved by sending notifications to disparate email addresses that use multiple SMTP relays.

If WhatsUp Gold allowed configuration of multiple outbound SMTP relays, then part of the alert definition for an email alert would be to select which configured SMTP relay to use for the alert.  This might work even better than a built-in relay as it would allow standard notifcations to go through one relay and 'escalated' alerts to be sent through a different relay.