This is how I understand Reset means under Maintenance:
When Outlook Express uses IMAP the mail messages and headers are stored on the mail server. However, a copy of the headers and (depending on the settings and whether you have read the message) the message is stored on your own computer - this is called the "local cached copy". This means that you can read messages you have previously read, even if your computer is no longer connected to the network.
If something should go amiss with Outlook Express, for example: it loses the dates on messages, you may need to delete your local cached copies of messages and headers, and re-load them from the mail server; use the following procedure:
1. From your Outlook Express menu, select Tools and then Options....
2. Select the Maintenance tab.
3. Click on the Clean Up Now... button.
4. In the File Information area you will see Local file(s) for: with a default of Outlook Express. If you leave this default, you will re-set all Outlook Express folders. To re-set one folder, click on the Browse... button, and select required the folder from the dialogue box, and click OK. Click the Reset button to reset the folder. [To do more than one folder, repeat this procedure.]
5. When finished, press Close and then OK.
6. Select the folder (or folders) you have Reset to see the newly refreshed message headers.
But, again, it may very well reset the counter, but I don't see why.