Mail Server FQDN – NOQUEUE: reject: RCPT from mail.example.org: 450 4.7.1: Helo command rejected: Host not found.
Mail server MTA (Mail Transport Agent) check incoming SMTP requests for the A Resource Record or CNAME which is registered with the authorized DNS server, the condition being that the mail server FQDN sent with EHLO/HELO is based on the A (AAA) or CNAME Resource Record must show.
NOQUEUE: reject: RCPT from mail.example.org[203.0.113.32]: 450 4.7.1 <mail.server.net>: Helo command rejected: Host not found.Exchange server FQDN incorrectly
Exchange mail servers that are configured for direct delivery must specify the FQDN valid in the DNS (mail.server.net) in mail flow (message flow) in send connectors under scoping (area definition) FQDN in HELO/EHLO.
ECP: mail flow -> send connectors -> scoping -> FQDN
Checking the configuration mxtoolbox.
Remarks
This article describes the bounce error, which causes EHLO to send a different FQDN when incoming messages are discarded. The result is „Helo command rejected Host not found“. Mail servers that are configured for direct delivery must specify the valid FQDN (mail.server.net) in the DNS for mail flow for send connectors under scoping for HELO/EHLO.
The value specified here is sent to the recipient’s mail server during the SMTP connection using the HELO/EHLO command. At this point, the remote server will then perform a reverse DNS record lookup (PTR) on the IP address you are connecting from and attempt to match the two values. If they don’t match, many mail servers give the message SPAM points or reject the message entirely. To prevent this, one must ensure that the DNS is set up correctly for the IP address(es) that will be selected for outbound delivery.
Example of the mail server that was set up with the IP address 203.0.113.32.
IP – Host Name
203.0.113.32 mail.example.org
The following must be the case.
mail.example.org -> must be resolved -> 203.0.113.32
203.0.113.32 -> must be resolved -> mail.example.org
The HELO/EHLO name should be the fully qualified domain name (FQDN) of the system.
if email is sent from the IP address 203.0.113.32, and mail.example.org resolves to 203.0.113.32 (and the DNS PTR for 203.0.113.32 is set to mail.example.org), mail.example.org can be used as value for HELO (EHLO).