Ask your recipients if they're receiving attachments called "winmail.dat". If
so, here's what MS says about this:
---quote---
The Winmail.dat file is used to preserve formatting that the sending client
includes in the message, but is not recognized by the receiving client. In the
case of Outlook, the Winmail.dat file includes Rich Text Formatting (RTF)
instructions. This type of formatting is used with the Microsoft Outlook Rich
Text Format and when using Microsoft Word as an e-mail editor.
---end quote---
You may be replying to their e-mail in a plain text format which eliminates this
condition.
Here are a few articles on this subject:
Winmail.dat attachments are included in received messages in Outlook 2000
http://support.microsoft.com/default.aspx?scid=KB;EN-US;Q197064
How to Prevent the Winmail.dat File from Being Sent to Internet Users
http://support.microsoft.com/kb/138053/EN-US