Unable to correct problem with my iCloud Mail

Hey there sapper,

If you place your mouse/cursor to the right of the scroll bar, do you see an option to adjust the columns? If so, go ahead and try adjusting their size. You'll also want to do the same by placing your mouse/cursor to the right of your emails to try adjusting that column as well.

If this persists, try closing the web browser window and clear your website data just in case this is an issue with the website, then test the site again. To clear the data, use these steps:

In the Safari app  on your Mac, choose History > Clear History, then click the pop-up menu.

Choose how far back you want your browsing history cleared.

Clear your browsing history in Safari on Mac - Apple Support

Take care!