MS-Exchange with insufficient resources to receive mail

As discussed in this article, in cases where an AVAS Maildrop does not accept emails with a temporary 4xx code, they will be queued on its MX for 5 days.

One of the reasons why emails may not be accepted is due to insufficient resources in Exchange, DNS issues, or the Exchange has been hacked.

As with all emails rejected by the Maildrop, in the Incoming Mailaudit, emails marked with a ‘?’ in Content can be seen, along with a message similar to the following in the SMTP Response:

451 4.7.0 id=05642-14-3 – Temporary MTA failure on relaying, from MTA(smtp:[XXX.XXX.XX.XXX]:25): 451 4.7.0 Temporary server error. Please try again later. PRX5

Another similar message is

452 4.3.1 Insufficient system resources

or also

451 4.7.0 Temporary server error. Please try again later. PRX2

or also frequently in Office365

451 4.4.4 Temporary server error. Please try again later ATTR5

These cases usually indicate that the email is being held back due to Insufficient System Resources on the part of Exchange or temporary issues with Office365.

Depending on the type of rejection, it may also happen that the Mailaudit information is incomplete, which is a clear sign that there is a failure in the Maildrop that is not fully accepting the emails.

It can be confirmed that it is an Insufficient System Resources error by checking the Exchange logs. This is something that can be diagnosed and must be resolved by the Exchange administrator.

More information on this common issue can be found on Microsoft’s site for Exchange 2016, as well as this updated resource monitoring guide for Exchange 2019 (https://docs.microsoft.com/en-us/exchange/mail-flow/back-pressure?view=exchserver-2019).

Note

These sites also explain how to resolve Back Pressure issues in Exchange 2007, and Back-Pressure in Exchange 2013.

Warning

As with all cases where emails are queued due to being rejected by Maildrop with a 4xx error code (starting with 4 indicates that the rejection is temporary and should be retried later), the AVAS MX queues the emails and continues attempting delivery until the Maildrop responds or for the next 5 days. If the 5-day retention policy is exceeded without the MS-Exchange administrator resolving the issue, or without the Exchange installation recovering resources on its own, those retained emails will bounce back to the sender.

It should be noted that these 4xx codes from the recipient (Exchange) indicating that it does not have the resources to accept more emails, lead to the AVAS MXs retrying and spacing out the retries to avoid being classified as Spam or DDoS servers, which can lead to delivery delays of several hours.

We recommend a monitoring system that generates alerts in MS-Exchange to inform administrators in a timely manner, allowing them to check if there were moments of resource scarcity.

Last Updated on 2024-08-21