Can't send emails to addresses outside Outlook.

I have an issue with Outlook, sending emails to anyone with a non-Outlook email address. (Gmail for example) I CAN receive every incoming email without problem. However after clicking the Send button, the emails gets returned via email address removed for privacy reasons with this error message: 

Remote server returned message detected as spam -> 550 5.7.1 [2a01:111:f403:2e0e::801 19] Gmail has detected that this message;is likely suspicious due to the very low reputation of the sending;domain. To best protect our users from spam, the message has been;blocked.

Tried different devices, with different internet connections, but got the same result.