SMTP Code: 451 4.4.1 [Internal] No valid hosts (unable to make any connections)

Products

• Messaging Gateway

Issue/Introduction

Messages to one or more Domain routes are accumulating in the Messaging Gateway (SMG) delivery queue

To	From	Date	Route	Size	Last Error	Next Attempt	Total Attempts
[e-mail]	[e-mail]	June 19, 2026 8:22:05	Domain: domain.test	4232	451 4.4.1 [Internal] No valid hosts (unable to make any connections)	June 19, 2026 9:37:05	3

Environment

• Product: Messaging Gateway
• Version: All

Cause

The "Domain: domain.test" entry for the SMTP route indicates that the message is trying to be delivered to a domain specific route defined in Protocols > Domains but the configured destination servers for the domain route are not accessible. 

Resolution

1. Go to the Protocols > Domains page and open the entry for the domain that cannot be delivered
2. Select the Destinations tab
   
3. Connect to the admin command line on the SMG scanner
4. Attempt to telnet to the IP or hostname defined as a destination host for the domain
   telnet 10.253.38.101 25
5. If the telnet connection cannot be established to the destination server the issue is one of the following:
   • If the telnet attempt immediately fails, the destination server is reachable but is rejecting connections to the SMTP port
   • If the telnet attempt waits and then times out, the destination server is offline or unreachable
   • If the telnet attempt returns "No route to host" then the default router does not have a route to the destination server's network
   • If the telnet connects but no 220 SMTP banner is returned, the destination server is running but not able to accept email

In each of these cases the issue lies with either the destination server, the intermediate network, or an intermediate firewall and should be investigated by the network administration team.