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.
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
- 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
- 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.
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.