I have a rule that assigns a category to sent messages. It works in Outlook
2007 and I believe it worked in Outlook 2003 also -- can't say for earlier
versions. Here is the wording:
Apply this rule after I sent the message
with 'domain.com' in the recipient's address
assign it to the <Category> category
and stop processing more rules
Of course you will need to change "domain.com" to some part of the
receipient's e-mail address, and add the category name you want to assign the
message to.