API: Message relay / release

This enables a SMTP server to be configured, and messages to be manually "released" via the relay server. Aditionally, messages can be auto-relayed via the SMTP server do Mailpit acts as a form of caching proxy.

@see #29

utils/tools/message.go

@@ -0,0 +1,59 @@
+// Package tools provides various methods for variouws things
+package tools
+
+import (
+	"bufio"
+	"bytes"
+	"net/mail"
+	"regexp"
+
+	"github.com/axllent/mailpit/utils/logger"
+)
+
+// RemoveMessageHeaders scans a message for headers, if found them removes them.
+// It will only remove a single instance of any header, and is intended to remove
+// Bcc & Message-Id.
+func RemoveMessageHeaders(msg []byte, headers []string) ([]byte, error) {
+	reader := bytes.NewReader(msg)
+	m, err := mail.ReadMessage(reader)
+	if err != nil {
+		return nil, err
+	}
+
+	reBlank := regexp.MustCompile(`^\s+`)
+
+	for _, hdr := range headers {
+		// case-insentitive
+		reHdr := regexp.MustCompile(`(?i)^` + regexp.QuoteMeta(hdr+":"))
+
+		// header := []byte(hdr + ":")
+		if m.Header.Get(hdr) != "" {
+			scanner := bufio.NewScanner(bytes.NewReader(msg))
+			found := false
+			hdr := []byte("")
+			for scanner.Scan() {
+				line := scanner.Bytes()
+				if !found && reHdr.Match(line) {
+					// add the first line starting with <header>:
+					hdr = append(hdr, line...)
+					hdr = append(hdr, []byte("\r\n")...)
+					found = true
+				} else if found && reBlank.Match(line) {
+					// add any following lines starting with a whitespace (tab or space)
+					hdr = append(hdr, line...)
+					hdr = append(hdr, []byte("\r\n")...)
+				} else if found {
+					// stop scanning, we have the full <header>
+					break
+				}
+			}
+
+			if len(hdr) > 0 {
+				logger.Log().Debugf("[release] removing %s header", hdr)
+				msg = bytes.Replace(msg, hdr, []byte(""), 1)
+			}
+		}
+	}
+
+	return msg, nil
+}