Cant see mail content after High Sierra

Since updating to High Sierra, I've had issues with my email.

The messages are downloading to the mail accounts (iCloud, plus 2 other accounts), but the message content doesn't display on the preview pane when the message is selected (or takes several minutes to appear). The message content generally loads ok on other Apple devices, but refuses to do so consistently on my Macbook Pro, rendering the mail program essentially useless.