add vendoring

vendor/github.com/emersion/go-smtp/smtp.go

@@ -0,0 +1,30 @@
+// Package smtp implements the Simple Mail Transfer Protocol as defined in RFC 5321.
+//
+// It also implements the following extensions:
+//
+//  8BITMIME: RFC 1652
+//  AUTH: RFC 2554
+//  STARTTLS: RFC 3207
+//  ENHANCEDSTATUSCODES: RFC 2034
+//  SMTPUTF8: RFC 6531
+//  REQUIRETLS: RFC 8689
+//  CHUNKING: RFC 3030
+//  BINARYMIME: RFC 3030
+//
+// LMTP (RFC 2033) is also supported.
+//
+// Additional extensions may be handled by other packages.
+package smtp
+
+import (
+	"errors"
+	"strings"
+)
+
+// validateLine checks to see if a line has CR or LF as per RFC 5321
+func validateLine(line string) error {
+	if strings.ContainsAny(line, "\n\r") {
+		return errors.New("smtp: A line must not contain CR or LF")
+	}
+	return nil
+}