To ensure remote mail servers reliably accept SMTP connections from the Sendio ESP, first determine the public IP address associated with it and then ensure a corresponding PTR record is set up for this address. To determine the public IP address, access the Console (SSH) Interface > Network Diagnostics > SMTP Diagnostics Test. This server will respond with the public IP of the Sendio ESP and with the associated name. Below is a sample output of such a connection:
Connecting to smtp2.google.com:25…
><-220 smtp.google.com ESMTP?
><-250 smtp.google.com Hello esp.sendio.com [126.96.36.199], pleased to meet you?
>–Received a 2XX code. Response is OK: (220 smtp.google.com ESMTP)
><-221 2.0.0 smtp.google.com closing connection?
The IP address is showing as 188.8.131.52 with a corresponding hostname (via DNS reverse lookup) of “esp.sendio.com”. If the name that appears to the left of the IP address is not associated with your domain, a PTR record needs to be defined. Another way to test reverse DNS lookup for an IP is by using http://www.mxtoolbox.com. In most cases, you will need to contact your Internet Service Provider (ISP) and request them to set up an rDNS so that your public IP address translated back to the machine name of your Sendio ESP (e.g.”sendio.yourdomain.com”).