Unable to send message;Reason:554 5.1.8 Email Outgoing Blocked.

Hi,

For the last few hours, I've had a problem sending emails.

Whenever I try to send an email, I get a message saying "Unable to send message;Reason:554 5.1.8 Email Outgoing Blocked."

I've tried sending to several addresses but always get the same warning message.

I tried clicking on the learn more button to try to understand what the problem is but couldn't find any explanation.

Any idea what the issue could be?