Hi Madalina,
Thanks for your reply in the private message.
I have checked the problematic account and I notice that the problematic account is not an account of the Office 365 Business Plans. I also checked the MX records for the problematic account and all MX records seems to be pointed to the on-premise Exchange servers. Given the situation, we suggest you contact the person who help you set up all accounts before.
Your understanding is highly appreciated.
Best Regards,
Erick