Kevin,

SpamFilter will queue emails if the destination SMTP server is not available or is not responding to connection requests, and will retry delivery every few minutes until the server is back online.

If the destination server is available and is responding to SMTP traffic, but rejects emails due to errors, then this error conditions should result in Exchange erroring out connection requests with SMTP error codes. These error codes can be in the 400-499 range, indicating a temporary error condition, or in the 500-599 range, indicating a more permanent condition.

Per RFC, servers are supposed to retry delivery for a reasonable amount of time if the errors are in the 4xx range, and give up and notify the sender with a NDR (non-deliverable email report) if the errors are in the 5xx range.

SpamFilter does follow this behavior by default, even though this default behavior can be changed with these two parameters in the SpamFilter.ini file:

;Determines if SpamFilter should hold in the queue emails that were rejected by the destination SMTP server with an error in the 4xy range
QueueIfDestinationError400=true

;Determines if SpamFilter should hold in the queue emails that were rejected by the destination SMTP server with an error in the 5xy range
QueueIfDestinationError500=false


What actually happened thus depends on how your Exchange server was rejecting emails that SpamFilter was forwarding to it. If you send us a few minutes worth of SpamFilter’s activity logfile when Exchange was down we’ll try to find out more.