Well, are we talking about:
1. First, you sending an e-mail including your signature, someone replies to
it, and now you are replying to him. When you do that, the original
signature from when you first sent the e-mail now shows as an X.
or
2. Any old e-mail and when you reply and insert your signature, it shows as
an X.?
The first scenario is certainly understandable, if that's it. E-mails go
through lots of squishing and squashing (to be non-technical!

when send
and replied, etc.
I think I would need a more detailed explanation of the steps involved to
get the X, if it's something that always happens under the same
circumatances.
You might also want to post your signature. I could look at the HTML and see
if it's screwy. How did you create your signature? Did you just use
Outlook's user interface? Do you know how to edit the HTML signature file
directly? We might need to do that!