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Outlook 365 - Not recognized as a valid sender

I've got a domain (sariftech.com) and an outlook 365 account (martin@sariftech.com) and more recently I've built a website through godaddy's website builder. I'm not 100% sure about timing but it may have been the website that has caused it but I'm no longer able to send emails for some reason, instead I'm getting the following error back when using both Outlook and the Outlook365 website:

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 messages outside of your organization. Contact your email admin for assistance.

Thanks