Some additional repos to consider:
mailx — A lightweight SMTP mail library
zeffrin/go-mail — Email library for gocode. Never got around to doing MIME support.
mailaddress — Mail address parsing library for Go
zohosendmail — A small library to send a mail using the zoho.net mail api.
fastbill/go-mail — Golang mail library with interchangeable backends
Once a week...... I send out a list of most interesting Go libraries and apps.
Want to get it?