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Why has e-mail bounced with the error: domain exceeded the max defers and failures per hour?
Max Defers Mechanism
Is a server defence against spam abuse that tracks and limits outbound mail based based the maximum percentage of failed or deferred messages sent by a domain per hour.
The system monitors how many emails have bounced during the past hour against how many emails have been sent out, and if a domain exceeds the maximum bounce limit, it will not be able to email until a pre-set time has passed and the domain is back within limits.
Our shared servers are set to a failure ratio of 75%. The tool is hardcoded to track deferrals/failures after 5/5 have been reached for the past hour.
Once the limit and 75% failure ratio have been hit, ALL mail for the domain will fail until the ratio drops below 75%. This will happen after the earliest failure(s) roll off the beginning of the hour.Tracking & Bounces
Percentage counts do not start until there have been 5 defers or failures. This is what the (x/5) is indicating in the bounce message. The measurement of defers and failures kicks in once 5 defers or failures have occurred. So, if a user sends out 4 emails and 4 of them are deferred, we would see a (4/5) in the logs. If you have a 75% threshold on suspending email sending, even though 100% of this user’s messages have failed, that user will still be able to send email because a total of 5 messages haven’t bounced during that hour yet. The percentage of failures kicks in after 5 total failures, and then allows up to a 75% failure rate for the rolling hour.
IMPORTANT NOTE: If your mailbox receives a lot of junk email, and you have an Auto Responder set up, then that can trigger a max defers block if a high percentage of the auto responder messages bounce back. Reducing the amount of spam being accepted by your mailbox, such as lowering the detection theshold in SpamAssasin may help, or in sever cases, you may need to disable or remove the auto-responder from that mailbox.Other Examples:
Say you are trying to send out 60 emails, one each minute. If the first 5 emails go through without an error and then the next one fails, that would be the first failure. So in the first 6 minutes you have 1 failure. This earliest failure will mark the rollover of the next hour.
The next 10 emails go through fine, but then 4 emails after that fail. At this point, you have reached the 5 failure limit. It has now been 20 minutes altogether. However, the first failure was 6 minutes in. The domain sending out the emails would not be able to send any further email until an hour has passed since the first failure. In this case, you would need to wait 46 minutes to send another email, as the first failure would drop off at that point, leaving you with 4 failures in the past hour.FAQ
1. For the hourly limits, is this a set hour (i.e. 3:15 to 4:15pm)?
No, the “per hour” means “in the past 60 minutes”. When a defer or failure happens, if there are additional failures in the past 60 minutes, +1 is added to this count. The ‘hour’ is based on the earliest failure (that keeps it over 5/5).
2. If I send 1 message in the hour, and it fails, does that mean all my other messages in the hour also fail?
No bounces until 5/5 messages (for the hour) are reached, from that point on it’s 75% of the messages they send. If they are over 5/5 and 75%, further messages will immediately bounce and drop (they will not be re-queued), even if they are good messages.
