Generally, emails are delivered directly to the destination server. However, if the delivery attempt to the destination server returns a temporary failure, all email messages sent to known, valid recipients are queued locally on the filtering servers for delivery retry - see View Incoming Delivery Queue.
Emails which have been permanently rejected by the destination server with a 5xx error code, will NOT be queued and are rejected by the system - you can see these messages in the Spam Quarantine.
Automatic Retry Schedule
Messages queued for known valid recipients because of temporary problems with the destination route (e.g network problems) are automatically retried for delivery at the following approximate intervals:
- During the first 2 hours, delivery is retried at an approximate interval of 15 minutes
- During the next 14 hours, delivery is retried incrementally, starting at 1 hour and multiplying by 1.5 with each attempt
- From 16 hours, until 60 days have passed since the initial failure, delivery is retried at an approximate interval of every 6 hours
- After 60 days we generate a bounce to the sender. If the bounce cannot be delivered immediately (i.e. if the 'message could not be delivered' message (Non-Delivery Report) fails to send), it will be frozen automatically. After this time, delivery of the message will have permanently failed
When a message is frozen (it cannot be delivered to the recipient or returned to the sender), no more automatic delivery attempts are made. An Admin user can “thaw” ( force retry) such messages when the problem has been corrected.
Spam Experts caches valid recipients. After the cache expires, Spam Experts will not queue email for those recipients. The message will be temporarily rejected so those will be queued on the sending server. The sending server in this case will automatically retry delivery. When using the Mailboxes list (Mailboxes Overview/Configuration), no caching is involved and Spam Experts continues to accept and queue the emails for all specified recipients.
The SMTP RFC 5321 specifies a sending server must queue messages which cannot be directly delivered because of a temporary failure at the receiving end. Therefore in the case of temporary issues with the email infrastructure, emails will not be bounced immediately but are instead queued on the sending server(s) and automatically retried for delivery. In case of downtime of the destination mailserver, messages are only accepted for delivery by the filtercluster if the recipient is known to be valid. Valid destination recipients are cached (when “Reject email to mailboxes not listed in the Mailboxes tab" is enabled), per filtering server.
Your own Fallback Server(s)
Please note that when you specify multiple destination routes, Spam Experts assumes you run your own fallback system. If the specified fallback server is not responding to recipient callouts, there will be no database built up of valid recipients internally. We recommend not to specify any fallback server(s) unless you've specifically designed your infrastructure to handle outages of the main destination server.