Hi barneyhairball,
Welcome to the Apple Support Communities!
I understand that you are receiving a SMTP error while trying to send an email from your iCloud account. In this situation, I would recommend reading over and working through the suggested steps in the attached article, specifically the If you receive an alert when you send a message from your @icloud.com email address using Mail on your Mac section.
Get help using iCloud Mail - Apple Support
I also notice by the time and date of your post that some services were experiencing a disruption during that time. As an added resource, please use the next attached article to check the status of Apple Services, Stores, and iCloud in the future.
Apple - Support - System Status
Best regards,
Joe