I forgot, have you already opened port 587 through the firewall?
The same within the network you do.
Networking/Firewalls/Create Firewall
1 Name (example: Firewall of my forum)
2 New rule, select “Custom”.
3 Custom
4 TCP
5 587
6 Custom
7 UDP
9 587
9 Select the dropplet where you want aply the rules.
And finally click on “CREATE FIREWALL”.