The characteristics of the subscribe2 emails are:
* not TO or CC to the user; but BCC
* precedence: bulk
* contains at least two links
* contains "unsubscribe" verbage
Although subscribe2 is a benign plugin, the emails closely match those of actual spam. There's little I can do to work around that, short of delivering directly to each individual recipient. I won't do this, though, because I know of at least one site with over 300 subscribers. Since I host that site, I don't want Apache + PHP generating three hundred individual emails on my server.