Generally emails are directly delivered to the destination server and not stored locally on the filtering machines. However if the destination server is unavailable, all email sent to known destination recipients are queued locally on the filtering servers for delivery. Emails which have been permanently rejected by the destination server with a 5xx error code, will NOT be queued and are rejected by the systems. We have a list available with SMTP Error Codes available in our knowledge base.

Queue Access

You can access the email queue from the webinterface, from which you can also manually force-retry delivery of a queued message.

Automatic Retry Schedule

Messages queued because of temporary problems with the destination route (for example network problems) are automatically retried for delivery at the following approximate intervals:

• During the first 2 hours, delivery is retried at a fixed interval of 15 minutes.
• During the next 14 hours, delivery is retried at a variable interval, starting at 15 minutes and multiplying by 1.5 with each attempt (e.g. after 15 minutes, then 22.5 minutes, then 34 minutes, and so on).
• From 16 hours since the initial failure, until 4 days have passed, delivery is retried at a fixed interval of every 6 hours.
• After 4 days we generate a bounce to the sender. If the bounce cannot be delivered immediately, it will be frozen. After this time, delivery of the message will have permanently failed.

Messages that are frozen because of more permanent problems with the destination route (for example, the hostname not existing) are automatically retried for delivery at the following approximate intervals:

• The first 21 days delivery is retried at a fixed interval of 3 days. After this time, delivery of the message will have permanently failed.

Messages Queued

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 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 up to 96 hours.

When a destination server cannot be reached for 4 days, all messages will be bounced after 4 days and no new email will be accepted/queued until the destination server is back online. This 4 day period is conform the SMTP RFC. The reason it's not longer than 4 days, is because it's important for the sender to be aware that delivery of their message has been failing for 4 days so they can try and contact the recipient in another way.