Network is unreachable (101) error with Outlook 365 mail

Hello,

for some reason SMTP settings work fine on my demo server but they don’t work on final/public server.

I’m getting this error “SMTP ERROR: Failed to connect to server: Network is unreachable (101)SMTP connect() failed.”

My settings:
SMTP host: smtp.office365.com
Encryption: STARTTLS
Port: 587
Authentication: Yes
Username: my-email
Password: my-email-password

Any idea what might cause this?