SMTP Code: "421 4.4.0 Unable to connect to DNS - try again later" when sending mail to Messaging Gateway

Products

• Messaging Gateway

Issue/Introduction

When sending an inbound message to a Messaging Gateway, the peer gets an error response of "421 4.4.0 Unable to connect to DNS - try again later"

Similar error messages to those below are found in the maillog:

2011 Jun 29 10:34:12 UTC (warning) ecelerity: [23186] sieve: RDNS: RDNS lookup for connecting IP <IP_ADDRESS> returned SERVFAIL
2011 Jun 29 10:34:12 UTC (info) ecelerity: [23186] ML-REJECT: Rejection on: <IP_ADDRESS>:25,200, sent to host: <IP_ADDRESS>:49653, Audit ID 00000000-x0x0dxx000000a00-00-0x0x0x0000x0, 421 4.4.0 Unable to connect to DNS - try again later
2011 Jun 29 10:34:12 UTC (notice) ecelerity: [23186] ESMTP-02739
2011 Jun 29 10:34:12 UTC (info) ecelerity: [23186] ML-HOST_DISCONNECTED: <IP_ADDRESS>:49653 disconnected. (ID 90656f58)

Cause

This issue can occur when you enable any of the DNS Validation features under Protocol → Settings, and there are problems with the configured DNS server(s).

Resolution

Ensure that the configured DNS server is working and is not overloaded. Navigate to Administration → Configuration → host → DNS/Time.