S/MIME or Secure MIME, is a alternative method of securing e-mail. Instead of a web of trust model of verifying the identity of a recipient, S/MIME relies on the Trusted Certificate Authority model, similar to the security model for SSL.

K-9 Mail does not currently support S/MIME, however forks of K-9 such as SMile have implemented it, so there is ongoing work to see S/MIME incorporated into the code base.