A user asks: "When I set a flag for a sent item it does not show up in my reminders dialogue box however my received e-mails do when I set a flag. Am I doing something wrong?"
Flags only fire from default folders – Inbox, Calendars, Contacts, Tasks etc. Flags in any other folder won't fire unless you use Reminder manager from www.slovaktech.com.
You can not set Quick Flags on items you send at the time of sending (Outlook 2003). You'll need to set them after the message is sent. This is to prevent "flag spamming". If you need to set flags for your own use at the time of sending, try Sperry Software's Follow-up reminder (http://www.sperrysoftware.com/Outlook/Follow-Up-Reminder.asp)
Update: Reminders will fire from all data files in an Outlook 2010 profile as long as the data file is configured for indexing.