Antispam just hate emails that are html-only.
I acknowledge that 'newseletter' allows for newsletters in plaintext+html to be sent. However, the notifications automatically sent when people subscribe (or otherwise interact with the plugin) seem to lack that feature. They seem to be html only.
So people register, the html-only confirmation goes in spam, and they never get the newsletter that so diligently comes in plaintext.
That makes me a sad panda.