m := goemail.New()
m.From = "[email protected]"
m.To = `Bob <[email protected]>`
m.Bcc = `"Eve the eavesdropper" <[email protected]>`
m.Subject = "Secret Plans"
m.Body = "<b>Attack at Dawn!</b>"
m.AddHeader(header.MessageID("<[email protected]>"))

// header validation
err := m.Validate()
if len(err) > 0 {
    // handle errors
}

raw := m.Raw()
// use in gmail api, aws ses etc

Custom email

headers := []header.Header{
    header.MIMEVersion{},
    header.Address{Field: header.AddressFrom, Value: "[email protected]"},
    header.Address{Field: header.AddressTo, Value: "Bob <[email protected]>"},
    header.Subject("Secret Plan"),
    header.MessageID("<[email protected]>"),
    // insert own header that satisfies header.Header interface
}

text := mime.NewEntity(
    []header.Header{header.NewContentType(
        "text/plain",
        header.NewMIMEParams("charset", "us-ascii"),
    ),
    }, "Attack at Dawn!")

html := mime.NewEntity(
    []header.Header{header.NewContentType(
        "text/html",
        header.NewMIMEParams("charset", "utf-8"),
    ),
    }, "<b>Attack at Dawn!</b>")

alt := mime.NewMultipartAlternative(headers, []*mime.Entity{text, html})

// header validation
for _, h := range alt.Headers {
    if err := h.Validate(); err != nil {
        // handle error
    }
}

// generate raw email output, use in gmail api, aws ses etc.
raw := alt.String()

Features

General

email header validation
non us-ascii support for header and mime parameter values
non us-ascii support for email body

Folding

header (78 octet limit)
RFC 2045 base64 (76 octet limit)
support for folding priority

Highly customisable and extensible

header.Header interface
folder.Foldable interface
standalone syntax checking library
standalone folding library

Relevant Documents

Package makes best effort to conform to following standards:

RFC 5322 — Internet Message Format. Specification for emails.
RFC 2045 — MIME Part 1: Message Body Formatting. Supports non-ascii body including attachments.
RFC 2046 — MIME Part 2: Media Types. Text, image, video, audio, application, multipart and message.
RFC 2047 — MIME Part 3: Message Header Formatting. Supports non-ascii headers.
RFC 2049 — MIME Part 5: MIME conformance.
RFC 2231 — MIME Parameter Value and Encoded Word Extensions. Supports non-ascii header parameters.
RFC 2183 — Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field.
RFC 5321 — Simple Mail Transfer Protocol. Imposes some length limits on various parts of message.