People get Security Alerts when opening my messages

I’ve heard from the people I send messages to that they get a security alert (see example below) when they go to open my messages. I’m using OE on XP Pro SP3, and when I open one of my own messages in my Sent Items folder, I get the same security alert. I’ve looked at my certificate properties in CSE, and I don’t see any indication that anything’s wrong. Also, on the Certification Path tab, it says, “This certificate is OK.”

My certificate was issued by COMODO.

Security Warning

There are security problems with this message.
Please review the highlighted items listed below:

Message has been tampered with

Since the copies of messages in my Sent Items folder have never left my computer, they obviously haven’t been tampered with (unless possibly by other software). What is causing this, and how do I fix it?

Hi,

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