Is there an actual message body, or is the content sent as a text or html attachment, in addition to the invitation? If it's an attachment, it won't show inline unless a preference is changed. Try also different choices for View/Message Body As.

There is an actual message body. I've already tried all the View/Message Body As options to no avail.

The last person I had that suggested the details were not shown did not have the Lightning calendar addon installed and operating. Do you?

If you do not all you will see is an ICS attachment.

I have the Lightning calendar installed. To be clear, I have no problem seeing the ICS inline. I just cannot read the body of the email that the person typed out.

Use the config editor to change mail.inline_attachments.text to true See if that makes a text attachment appear.

I don't think you understand what I'm saying. I'm not trying to get a text attachment to appear. I'm trying to get the actual body of the email to appear. All that I can see in the body of the email is the Event Invitation. I tried what you said anyway, but it did not work.

forward me one, matt_au at gmx.com and I will have a look, then I can not help but understand.

Forward as attachment (down arrow beside he forward button).

Just forwarded it to you. Also sent you a screenshot of what I can see. As you will see, you will be able to see the body of the email, but on the screenshot, all you can see is the Event Invitation. Thanks for looking into this.

As I said in my email, use the show all body parts addon. Make sure you set the "view message body as" on the view menu to "all body parts."

Unfortunately, this workaround doesn't work for me. Not sure why. Thank you for trying to help me, though. I truly appreciate it.