Do you have the Lightning calendar addon enabled? It really sounds to me like you do not, so the attached ICS file is not being opened by the application to show you a formatted view of the invitation.

Alternatively an attachment named winmail.dat might be present. A sure sign of a poorly configured copy of outlook, usually by someone that does not even know that there is a product other than outlook.

Thanks! Lightning downloaded and problem solved. That was very helpful.

I was getting winmail.dat files when some of those companies were sending word or pdf files. I've bought a new Laptop and don't recall getting any since then. Maybe solved? If I get a windat. file in the future, is there something I can request from the sender that will send a correct file?

you could point your correspondent to Microsoft's page on the subject https://support.microsoft.com/en-us/help/290809/how-e-mail-message-formats-affect-internet-e-mail-messages-in-outlook

Additionally you might want to install this add-on. https://addons.thunderbird.net/en-US/thunderbird/addon/lookout-fix-version/?src=search

There may be some built in support for TNEF/Winmail.dat in V78, but it does not look high on the features list, so might not make it. We wil just have to wait and see.