Sort problem!!!

[Hai Trieu]

Dear,

I have a problem when reply or forward a mail in Microsoft
Outlook - Exchange Server 5.5:
- When I reply ( or forward ) a mail, Outlook prefix
automatically RE: ( or FW: ) before the original Subject.

Ex: "RE: Hi, "
has been prefixed to "RE: RE: Hi, "

So when I sort mail by subject, Outlook does not sort my
mail in continuos segment, that means mails with "RE: Hi,"
in one segment and those with "RE: RE: Hi," in another
segment far from the first segment.

Anyone can help me?
Is the problem in Outlook or in the Exchange Server 5.5?

Thanks in advance,
Hai Trieu