header.htm causing content violation

[Millie]

Hello,

We are using Outlook XP and have always sent outgoing
email in HTML text. Now we are receiving notices from one
of our intended recipient's server as follows:

From: (e-mail address removed)
To: (e-mail address removed)

File(s): header.htm, header.htm, header.htm, header.htm

Matching filename: *.htm*, *.htm*, *.htm*, *.htm*

There were no attached files in the email sent to the
recipient. What is a header.htm? Is there some way to turn
it off? Why won't the recipient's server accept our emails?

Thank you for your help,
Millie