Hi
This is kind of old.. but any way.
I had the same hardware before and worked out of the box with gmail
smtp : smtp.gmail.com
security: starttls
port: 587
What to look for:
root@zs scripts> grep -r smtp ./
./sendmail:smtp-cli --subject="$SUBJECT" --from="$SMTPSENDER" --to="$RECIPIENT" --server="$SMTPSERVER" --port="$SMTPPORT" $SECURITYSTRING $AUTHSTRING --body-plain=$BODY
./alerts_start: echo smtp.gmail.com > $CONFIG/EMAIL/SMTPServer
root@zs scripts> smtp-cli --version
smtp-cli version 3.6
root@zs scripts> man smtp-cli
No manual entry for smtp-cli
Seem that you need to google for smtp-cli.
On the other hand I would suspect some firewall or route issue.
Try to telnet from your zs to local server.
https://www.port25.com/how-to-check-an-smtp-connection-with-a-manual-telnet-session-2/