does the mail have a winmail.dat attachment?

No, there is no winmail.dat attachment (at least by the time it gets to me) The emails that cause problems do come from a Microsoft system and contain these lines (in case they are relevant):

    X-MS-Has-Attach: 
    X-MS-TNEF-Correlator: 

The problem messages also went thru a Kaspersky filter at the sender

    X-KASflt

and was evaluated by Barracuda Networks. Perhaps the message was damaged during one of these operations. Thanks for looking into the problem

Picking random MS specific X headers is not going to fix this.

What follows the header. Is there a body?

I ask about the attachment, because it is possible to screw up the settings in Outlook to send a winmail.dat file as an attachment instead of the body of the mail. The higher to person sending mail is in the other organization is, the more likely they are totally inept when it comes to using their technology.

The structure of the email as shown in View / Message Source is:

- all the headers

This is a multi-part message in MIME format.

Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

- the body of the message as text

Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

- the body of the message as html

Content-Type: text/calendar; method=REQUEST; charset="utf-8" Content-Transfer-Encoding: base64

- unreadable text

This is the entire message.

Can you suggest something specific that I can look for in the Message Source that would explain why it is not displayed? Or is the problem more likely due to something that was lost in

transmission, in a spam or virus filter, etc ?

If it is useful, I can remove private information from the email and send the rest to you.

Thanks for helping to resolve this.

Eric

Install lightning is my guess. So Thunderbird has a calendar.