HTML Email Can't Be Read As Plain Text??

I'm using the ASPEmail component to send HTML email newsletters:

Mail.Host = "mail.myserver.co.uk"
Mail.From = strFrom
Mail.FromName = strFromName
Mail.AddAddress strTo, strName_In
Mail.Subject = strSubject
Mail.Body = strMessage
Mail.AltBody = "PLAIN TEXT VERSION"
Mail.isHTML = True
Mail.SendToQueue

The emails are coming through fine, though in Outlook the option to 'View as Plain Text' is greyed-out. This should let the user see the plain text version as defined in the code above, but it is inactive.