GroupWise 2014 R2
When setting up the GroupWise Internet Agent (GWIA) and populating the setting "Relay Host for Outbound Messages" with a hostname instead of an IP address -- 450 Host Down can occur in a couple of situations.
In most instances, the likely cause is simply the inability of a GWIA to communicate directly with Relay Host (whether it is by using an IP address or hostname). This can be tested simply by creating a telnet connection from the GWIA to the mail relay.
To accomplish this open a terminal screen on the GWIA server on Linux or a Command Prompt on a Windows Server if GWIA is in the Windows environment and type in the following command:
telnet <relay hostname> 25 (Linux)
telnet <relay hostname>:25 (Windows)
telnet <IP address of relay> 25 (Linux)
telnet <IP address of relay>:25 (Windows)
If no connection information is returned for any of the options, then there likely is a router/firewall issue. If the connection works for the IP address but not the hostname, there is likely a problem with the "A" record of the relay server in DNS or DNS is not setup properly on the server itself.
Another possibility might be that a MX record was inadvertently assigned directly to the hostname. For instance: if the relay hostname is relay.mailserver.com and an MX record was assigned to that hostname directing it elsewhere, GWIA will use that entry first before testing it as an A record. The MX record should be assigned to the domain name "mailserver.com" which will point to the A record of "relay.mailserver.com".