… and why it removes subject keywords.
When you reply to a reply (or forward a forward) Outlook strips the original RE prefix (or FW) and replaces it with a new one, to prevent multiple RE's on the subject line. Occasionally you'll notice it fails to remove the prefix and you end up with a long string of Re's. This is because the first Re contains a leading space. It's in the subject line as
" RE:", not "RE:"
Background: Outlook is programmed to remove any prefix containing 3 or less alpha characters, followed by a colon (RE:, FWD:, NET: etc). So, under normal conditions, Outlook will strip any prefix. Since the leading space is not an alpha character, Outlook leaves the prefix in place.
Tip: If you want to include a short keyword in the subject when sending mail, use a leading space, no colon, or surround it with brackets to insure the recipient's mail client won't delete it on a reply.
Outlook Express works in a similar manner, but may strip longer prefixes.
OLEXP: Text in the Subject Line Followed by a Colon Is Removed (MSKB, applies to Outlook also)
Published February 22, 2005. Last reviewed on January 15, 2013.