Hi,
I believe you may talk about Gmail, Yahoo or kind of web based email services. If you are check email through a browser then there is no connection with Mail shield. Mail shield is only valid, if you configured mail to be retrieved to your Computer using a email client like Outlook Express, Microsoft Outlook, Thunderbird etc. Those applications use your computer email port to retrieve or send emails through email port. avast! shielded those ports and statistics will be shown if any emails go through that ports only. Else your statistics = 0.
Cheers...