enmime is a MIME parsing library for Go. It's built ontop of Go's included mime/multipart support, but is geared towards parsing MIME encoded emails.
It is being developed in tandem with the Inbucket email service.
API documentation can be found here: http://go.pkgdoc.org/github.com/jhillyerd/go.enmime
enmime is alpha quality: it works but has not been tested with a wide variety of source data, and it's likely the API will evolve some before an official release.
enmime is written in Google Go.
enmime is open source software released under the MIT License. The latest version can be found at https://github.com/jhillyerd/go.enmime