some emails arrive with "this message has no content"

Hey look-at-this,

We understand that some of your emails are showing as no content, and we want to help! Does this happen with multiple email accounts, or only one? Does quitting the Mail app, and relaunching seem to help: Quit apps on Mac

If not, rebuild the mailbox to see if the email content appears. Just in case, here's how: Rebuild mailboxes in Mail on Mac

We also recommend ensuring that your Mac software is updated: How to update the software on your Mac

Keep us posted on the results.

Take care.

It is because there is no message body in the email, whether the original author's intent, because the subject line was the message, or not by intent, as the message body only contained an image, or document attachment that inexplicably was removed during transit.