I noticed in the documentation that it cites RFC 4408, which RFC 7208 has superseded.

I cannot find anywhere else in the project that cites RFC 4408, but RFC 7208 is cited in #7 (comment) and #21 (comment). Comparing the trustymail code and these two RFCs, it seems that trustymail does not conflict with RFC 7208 and should cite it instead of RFC 4408.

If you would like, I can make this one-line change in the README and submit a pull request.