Inconsistant Email Formats

Hi all,

Okay...this is a weird one, or at least I think so.

Our office email is being hosted by our IP (Shaw Cable).
We all use Outlook2003 to connect to our pop3 accounts.  All PCs are setup to send in html format (for our signatures).
Most PCs are using WinXPPro, a few still on Win2KPro (All PCs have been updated fully (Win & MSO))

The Problem:
Emails sent from one machine are recieved in both html and plain text, depending on who recieved it.  It is not OS specific, as I have had both results on both OSs.  Of the people that recieve these emails in plain text, they recieve emails from other people in our office as html.  So this tells me that it isn't something on the recieving end.  But if the conversion isn't happening on the recieving end, then it must be on the sending end right......No, because some people do recieve these emails as html.  For testing purposes I was sending emails with no body, minimal body, no subject, a simple hello subject, attachments, no attachments, sending them to one person at a time, multiple people at a time.  This computer also recieves some emails as plain text that were sent as html.  If I send an email to myself on this machine, again, it recieves it as plain text.  There is no pattern to help me distinguish where this conversion is happening or why.  The only consistancy is that the emails are always recieved the same way each time I have tested.  As I said, all of the PCs tested have been updated (Win & MSO), all settings for sending and recieving in Outlook seem o be the same.  Everyone uses the same email filtering and AV software.  I have tried removing this persons signature and just sending a line of text, but is still recieved as plain text on these certain machines.  I am going crazy trying to figure this out.....any suggestions?