SMTP Dialog Reject error can show up based on 2 issues which are both related to connections between your host and the target host.
- Your outbound connection to your target mail address is not getting successful from your server because of the firewall and not supporting port 25 outbound connection.
- Your target mail address server is rejecting your connection to accept and your inbound receiving connection close response.
We suggest customers use this script port_checker.zip on their website server. Unzip this and access port_checker.php from the web URL. If you see port 25 opens on your server then your outbound connection is fine and this server is ready to scan. But also make sure that you do not have a firewall setup on SMTP. Which is very common on WHM Panel.
We do not have any solutions for inbound connection rejection as this solely depends on the server to whom you are connecting. They can either block your outbound connection to them based on your server’s poor IP reputations or improper mail server configurations. This includes things like SSL, SPF records, Domain Keys, DMARC records, rDNS/PTR records. To check your mail server health use this tool https://mail-tester.com