Repeated errors of this in /var/log/mail or /var/log/syslog are typically due to a network issue and are not Mailborder specific. The most common cause is a device or firewall in front of the Mailborder server that is performing either proxy connection or SMTP inspection.
- If you have a device acting as a proxy, you should configure it to pass the SMTP connections without proxy.
- If you have an inspection such as Cisco Firepower, you need to turn it off.
Mailborder uses TLS connections. Any device that tries to proxy or inspect this traffic is going to break that connection.