I have received an invitation. Thunderbird shows the text, but i see no date or anything similar. Checking the source of the mail shows, that there should be an .ics file attached. Why can't i see the attachment?
Thunderbird prevents the user to see a message whose structure is malformed. It looks like invitations sent by Microsoft Exchange Mailservers have such a malformed structure. The only way to make these "malformed" message parts visible is by using a plugin called Show all Body parts.
To install and use it take the following steps:
Open Thunderbird and choose Tools / Add-ons.
In the upper right corner you will find a search field named: Search all add-ons
Click into that field and search for show all body parts
Klick the install button.
To enable the plugin click on View / Message Body As / All Body Parts
You will now see the base64 decoded content of that ics file directly in the email. On the bottom you have also the ability to save this attachment as an .ics file (maybe you have to name it yourself as such!) which can then be imported into your own calendar (as long as it supports .ics files).