go-smtp：Go 编写的 SMTP 客户端和服务器库
An ESMTP client and server library written in Go.
Features
- ESMTP client & server implementing [RFC 5321]
- Support for additional SMTP extensions such as [AUTH] and [PIPELINING]
- UTF-8 support for subject and message
- [LMTP] support
https://github.com/emersion/go-smtp