2018/04/06 by Willem Stam.

New SMTP replies at Outlook: update your backoff patterns to avoid reputation damage

On April 5th, at 5AM CET, we were alerted by a huge spike in Outlook deferrals.
The chart below (copied from Postmastery Delivery Analytics) reveals the deferrals over time:

After investigating the issue, Postmastery delivery engineers discovered new SMTP replies at Microsoft domains. These errors are receiver’s side, not related to sender’s reputation.

Organization queue quota exceeded.
451 4.4.0 Message failed to be made redundant due to A shadow copy was required but failed to be made with an AckStatus of Retry
452 4.3.1 Insufficient system resources (TSTE)
452 4.3.1 Insufficient system resources (PrivateBytes)
452 4.3.1 Insufficient system resources (ProcessorTime)
SSL error: connect failed: protocol error while connected from
452 4.3.1 Insufficient system resources (UsedVersionBuckets[D:\Queue]
​​451 4.7.653 The mail server [xxx.xxx.xxx.xxx] has exceeded maximum number of messages per connection. (S3116) [VE1EUR02FT004.eop-EUR02.prod.protection.outlook.com]
​451 4.7.652 The mail server [xxx.xxx.xxx.xxx] has exceeded the maximum number of connections. (S3115) [VE1EUR03FT036.eop-EUR03.prod.protection.outlook.com]

Due to the fact that we saw lower number of ‘451 4.7.500 Server busy. Please try again later’, we assume this one has been replaced with the new ones. What’s good is that the new errors are more specific: ” rate limited due to IP reputation “, rather than “server busy, try later”.

​451 4.7.650 The mail server [xxx.xxx.xxx.xxx] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.​ ​
​451 4.7.651 The mail server [xxx.xxx.xxx.xxx] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.

By updating the backoff rules in PowerMTA, deferrals were reduced significantly. We advise other senders to do the same to avoid reputation damage.

​reply /​has been temporarily rate limited due to IP reputation/ mode=backoff

If you are using PowerMTA 4.5r6 or later then you can also use mx-rollup-list feature to consolidate Microsoft related domains into one queue. This will improve resource utilization. For example, Microsoft EU and APAC :

# Microsoft
mx eur.olc.protection.outlook.com outlook-eur.rollup
mx apc.olc.protection.outlook.com outlook-apc.rollup

Further assistance?

If you need any assistance, just contact our email delivery support desk at support@postmastery.net. Or use our contact form.