Your message couldn't be delivered because you weren't recognized as a valid sender

[MSCom]

Hello, I tried to send an email from my outlook account and got this information: Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.

What can I do to be able to send emails again?
Thanks