Hi Rjn,
I have checked the DNS record for your domain name, and find that the MX record is pointed to “ms25809609.msv1.invalid.outlook.com.helikonia.net” which is incorrect.
Given this situation, please delete the existing MX records for helikonia.net and www.helikonia.net, and then create a new MX record with the value ms25809609.msv1.invalid.outlook.com.
Thanks,
Archana