Hello there, PrimalUK.

It sounds like you may have been experiencing an outage while being unable to send mail using your iCloud account in the Mail app on your Mac. If you're still experiencing the outage, this Knowledge Base article offers up a checklist of steps for troubleshooting:

If you can’t send or receive email on your Mac - Apple Support

If you can’t send or receive email on your Mac

Follow these steps if the Mail app in OS X suddenly doesn't send or receive email.

If your Mac is connected to the Internet, but Mail no longer sends or receives email messages, the issue could be with the provider of your email service. Or you could have missing or outdated settings in the Accounts section of Mail preferences (Mail > Preferences).

If Mail refers to a problem with the mail server or network

Mail might say that it’s unable to connect because of a problem with the mail server or network. For example, the message might refer to a connection that timed out, or too many simultaneous connections:

If you're connected to the Internet but the connection timed out, your email provider could be experiencing a service outage. Contact them or check their status webpage to make sure that their mail service is online. Examples of status pages:

• iCloud mail status
• Gmail status
• Yahoo mail status
• AOL mail status
• Outlook mail status

If the message refers to the number of simultaneous connections, too many of your devices are checking your email account at the same time. Quit Mail on one or more of your other devices.

Thanks for being a part of Apple Support Communities.

Cheers.