The indication is that hotmail senses that the mail is spam. This could be that the sending email in smtp server doesn't match the email id in message, or possibly that the sending isn't authenticated. There are multiple reasons for error 550 5.7.1 .

I won't ask 'why only hotmail' or 'why only the last two days', but what can I do to change those settings?

Maybe if you post screenshot of your smtp server settings and your account settings, it may be possible to make sense of it. On your question, some email hosts are more rigorous in security checking than others. And the message you posted is explicit in stating that your email provider is on their block list. That may need to be discussed with your email host.

It is my provider. I got in touch with them and just got this reply: We sincerely apologise for any difficulty caused. As I checked the account, I discovered that this problem also affected other accounts. Our admins are aware of the problem and are already trying to resolve it. You'll be able to access email as soon as possible as the email problem is fixed.

Thanks for sharing that. Error codes, such as the 550 5.7.1, that you received are fairly explicit and I am pleased your host provider takes it seriously. This sometimes happens because multiple accounts share the same IP address, causing the sins of one user to be visited on others. I wish you well.