Troubleshooting: Handling SMTP error messages

Modified on Tue, 6 Aug at 12:16 AM

Problem

In this article, we will learn how to handle SMTP errors.

Error code
Description
How to solve/handle (PP side + Customer)

5.7.1 Message rejected as spam by Content Filtering

This is an SMTP error related to the Exchange server.

It is the customer’s policy; to disable it, we need the customer to run Get-ContentFilterConfig.
This will output their Content filtering config.

SCLRejectionEnable: true= rejection policy is enabled.
SCLRejectionThreshold: a value between 1 and 9. If their SCL rank is greater than this value.
AND
SCLRejectionEnable is set to true; their server will reject emails.

https://learn.microsoft.com/en-us/answers/questions/364279/message-rejected-as-spam-by-content-filtering.html

554, 5.7.105 SenderFilterAgent; Sender denied as sender's email address is on SenderFilterConfig list"

The sender was listed under the property “BlockedSendersAndDomains”

Get-MailboxJunkEmailConfiguration -Identity "user@domain.com"

To remove the sender’s email address from this property, use the following command:
Set-MailboxJunkEmailConfiguration "user@domain.com" -BlockedSendersAndDomains @{Remove="sender@otherdomain.org"}

STARTTLS extension not supported by server.

StartTLS is a protocol command used to inform the email server that the email client wants to upgrade from an insecure connection to a secure one using TLS or SSL.

We can disable StartTLS encryption through the backend.

Organization domains
Uncheck 'Use tls'

421, '4.3.2 The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel

This is a restriction on the Receive Connector in Exchange 2007. You will need to configure the receive connector to allow MaxInboundConnectionPerSource to a desired number. The default value is 20.

You can change the "Maxinboundconnectionpersource" to 100 to receive more connections from the sender's system.

Please try the command.
Set-receiveconnector - identity "connector name" -MaxInboundconnectionPersource 100

Allowing more connections will fix the issue.

For Exchange 2016, you may need to run this command first
Get-ReceiveConnector -Identity <connectorname> | fl MaxInboundConnectionPerSource

https://learn.microsoft.com/en-us/exchange/mail-flow/message-rate-limits?view=exchserver-2019

554, 5.7.1 Recipient address rejected: Rejected by SPF

The hard SPF-check happens during the SMTP-dialogue.

Allow list PP IPs

https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmgconfig_whitelist_overview

5.7.23 SPF validation failed.

The SPF check for example.com fails because of an incorrect or nonexistent SPF record on the sender side.

Change SPF checking mode
1. Log in to Plesk.
2. Go to Tools & Settings > Mail Server Settings.
3. Change SPF checking mode to less strict, for example, "Only create Received-SPF headers, never block".
4. Scroll down and press the OK button to apply the changes.

https://support.plesk.com/hc/en-us/articles/12377013705239

452, 4.3.1 Insufficient system resources (UsedDiskSpace[““\\Program Files\\Microsoft\\Exchange Server\\V15\\TransportRoles\\data\\Queue])'

The '452 4.3. 1 Insufficient system resources' error is originating from the Microsoft Exchange server due to the server either running out of disk space or free memory. To address this: Increase the amount of free space on the Microsoft Exchange server drive.

554, 5.4.14 Hop count exceeded - possible mail loop ATTR34

Email loop.

Check customer’s RULES (Exchange Online- Perception Point Redirect Rule).
Add custom header

421 Too many concurrent SMTP connections; please try again later.

This happens on a very busy mail server and requires an adjustment to Exim's configuration.

The setting to change is smtp_accept_max. By default on a cPanel server, the smtp_accept_max setting is set to 100.

You would want to increment in sets of 50, and if you need to set this high, use additional commands like netstat to see if there is a large number of connections from a particular IP. In that case, you may need to block IP addresses, but if there is an actual distributed DDOS attack a firewall or DDOS service may be needed to mitigate the attack. Exim doesn't have a way to inspect connections to determine if there is an attack.

The change to smtp_accept_max needs to be set in WHM under the Advanced Editor tab using the Exim Configuration Manager section of WHM. This ensures changes remain persistent when other changes are made in the future.

This is an example command to get SMTP connections to the server by the number of connections from the IP:

netstat -plan| grep :25 |awk {'print $5'}|cut -d: -f 1|sort|uniq -c|sort -n | tail -30

550, 5.7.64 TenantAttribution; Relay Access Denied

Wrong next SMTP server. For example: Using smtp.office365.com
instead of
domain-com.mail.protection.outlook.com
Using a Domain that is not accepted in the O365 workspace.

https://admin.microsoft.com/#/Domains

Solutions:

Chang the next SMTP field to their MX record (use MX lookup Tool)
Contact the customer

NDR (Email non-delivery-report)

550 5.5.0 Requested action not taken: mailbox unavailable
The recipient's <SMTP Address> domain is @hotmail.com or @outlook.com, and it wasn't found by SMTP address lookup.

The TXT record is responsible for 90% of the error. That is the first thing to look for (removed-RECOVCATION /unverified).

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