strip <style> in html emails

smtp/session.go

@@ -74,16 +74,15 @@ func (s *session) Data(r io.Reader) error {
 	files = append(files, attachments...)
 	files = append(files, inlines...)
 
-	email := &utils.Email{
-		MessageID: eml.GetHeader("Message-Id"),
-		InReplyTo: eml.GetHeader("In-Reply-To"),
-		Subject:   eml.GetHeader("Subject"),
-		From:      s.from,
-		To:        s.to,
-		Text:      eml.Text,
-		HTML:      eml.HTML,
-		Files:     files,
-	}
+	email := utils.NewEmail(
+		eml.GetHeader("Message-Id"),
+		eml.GetHeader("In-Reply-To"),
+		eml.GetHeader("Subject"),
+		s.from,
+		s.to,
+		eml.Text,
+		eml.HTML,
+		files)
 
 	return s.client.Send(s.ctx, email)
 }

utils/email.go

@@ -11,3 +11,27 @@ type Email struct {
 	HTML      string
 	Files     []*File
 }
+
+// NewEmail constructs Email object
+func NewEmail(messageID, inReplyTo, subject, from, to, text, html string, files []*File) *Email {
+	email := &Email{
+		MessageID: messageID,
+		InReplyTo: inReplyTo,
+		From:      from,
+		To:        to,
+		Subject:   subject,
+		Text:      text,
+		HTML:      html,
+		Files:     files,
+	}
+
+	if html != "" {
+		var err error
+		html, err = StripHTMLTag(html, "style")
+		if err == nil {
+			email.HTML = html
+		}
+	}
+
+	return email
+}

utils/html.go

@@ -0,0 +1,44 @@
+package utils
+
+import (
+	"bytes"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+// StripHTMLTag from text
+//
+// Source: https://siongui.github.io/2018/01/16/go-remove-html-inline-style/
+func StripHTMLTag(text, tag string) (string, error) {
+	doc, err := html.Parse(strings.NewReader(text))
+	if err != nil {
+		return "", err
+	}
+	stripHTMLTag(doc, tag)
+
+	var out bytes.Buffer
+	err = html.Render(&out, doc)
+	if err != nil {
+		return "", err
+	}
+
+	return out.String(), nil
+}
+
+func stripHTMLTag(node *html.Node, tag string) {
+	i := -1
+	for index, attr := range node.Attr {
+		if attr.Key == tag {
+			i = index
+			break
+		}
+	}
+	if i != -1 {
+		node.Attr = append(node.Attr[:i], node.Attr[i+1:]...)
+	}
+
+	for child := node.FirstChild; child != nil; child = child.NextSibling {
+		stripHTMLTag(child, tag)
+	}
+}