Outlook default mail form

I have created a customized form for reading incoming mail. I published the
form in my Personal forms library.
With Forms Administrator I have created a Reg- file to swap the standard
mail form to my customized form when reading mail.
I run the Regfile and after that all my mail uses the customized form.

Very simple, BUT when another user installes the same form, runs the same
Reg-file it doesn't work as it should! If the user reads an external mail it
uses the customized form. If he reads the same message another time it uses
the standard form! All mail in the Inbox uses the default form when reading
them.
If he reads messages in the Sent folder the customized form is used.
2 other users have tested the same thing and it works the same way for them
too.
Very strange!
Why doesn't Outlook use the customized form for all messages in the Inbox?

We all have Outlook 2002 SP3 and the mailboxes is in Exchange server