Over the last couple of years I’ve heard about problems people have had with Sanebox. There’s one I remember on this forum here.
Apart from technical issues, they, like most add-on email service companies, compile usage data from your emails (what people buy, for what prices, what they’re interested in, travel booking behavior, what groceries users buy, what brands they like, etc) and claim to appropriately anonymize and aggregate the data. (“to our subsidiaries or affiliates only if necessary for business and operational purposes. We reserve the right to use and share aggregated, anonymous data, which does not include any personal data, about our Service’s users as a group for any legal business purpose, such as analyzing usage trends and seeking compatible advertisers and customers.”)
It’s because of this that I do not consider Sanebox, or Edisonmail, or Spark: they all do this, and I’m uncomfortable with it.