Thank you Ken for your comment. I had actually read the articles about
Outlook attaching formating information in Winmail.dat, but I tested again
after your note just to double check myself. However, I believe that my
problem is a different one.

My custom form, that I use to create the MailItem, has user defined fields,
which, if I understand correctly, will force Outlook to attach the
Winmail.dat file, even if I send the message in a plain text format (I check
the MailItem..BodyFormat property in the MailItem.Send event and in addition,
my Outlook Tools - Options - Mail Format tab - Compose in this message
format is set to "Plain Text").

The problem is that if I attach a file to the message on the sender side and
send it through an SMTP server, the attached file will be embedded into the
Winmail.dat on the recipient side. (I can see this if I attach a plain text
file and open the Winmail.dat file on the recipient side in a text editor.
Besides the text of he attached file, Winmail.dat obviously has some binary
information.)

I don't mind that Winmail.dat is attached to the message on the recipient
side. However, I would like to have the file that was attached on the sender
side to show up on the recipient side as it was attached. (i.e not embedded
in Winmail.dat.)