Mail defaulting to wrong account to send

I can’t find exactly this problem answered on-line.

When I intend to send from my iCloud a/c I have to be vigilant or it may send from a seldom-used email address with the ISP. I want the default sending server to be the iCloud a/c .

How can I set this up, please?

You change change the ‘default account’ for Apple Mail

For MacOS — How to Specify the Default Account in macOS Mail

For iPhone/iPad this should get you there too — https://discussions.apple.com/thread/252209523?sortBy=best