For each domain you can customize destination SMTP servers, and, if you wish, also add additional SMTP servers for each in case the primary server is offline. This can be done only for individual domains, not for the default destination SMTP server listed in the SpamFilter's "Settings - Configuration" tab.

Please note however that if one or more destination SMTP servers for your customers are offline, this should not impact SpamFilter's functionality. If one or more of these servers is down, SpamFilter will place the emails for these domains in a queue, and will attempt re-delivery every 20 minutes (this value can be changed) until the server(s) are back online. The emails being delivered to the other domains are not affected, since SpamFilter will attempt delivery of these other emails immediately upon receipt, without having to go thru the queue directory first (which may hold tens of thousands of pending emails if one or more destination SMTP servers are down).

When SpamFilter attempts re-delivery of these queued emails, it will do so by placing a limit on the number of concurrent outgoing connections to the destination SMTP servers. This will prevent the overload of SpamFilter's server (and of the destinatino SMTP servers). This limit is equal to one fourth (1/4th) of the value you have configured for SpamFilter's "Max concurrent incoming SMTP connections". So for example, if that "Max concurrent incoming SMTP connections" value is set for 200 connections, SpamFilter will only use at most 50 concurrent outgoing connections when attempting re-delivery of the emails stuck in the outbound queue.