fix #943

irc/accounts.go

 	if config.DKIM.Domain != "" {
 		fmt.Fprintf(&message, "Message-ID: <%s@%s>\r\n", utils.GenerateSecretKey(), config.DKIM.Domain)
 	}
+	fmt.Fprintf(&message, "Date: %s\r\n", time.Now().UTC().Format(time.RFC1123Z))
 	fmt.Fprintf(&message, "Subject: %s\r\n", subject)
 	message.WriteString("\r\n") // blank line: end headers, begin message body
 	fmt.Fprintf(&message, client.t("Account: %s"), account)

irc/email/dkim.go

 	Version:               1,
 	Canonicalization:      "relaxed/relaxed",
 	Algo:                  "rsa-sha256",
-	Headers:               []string{"from", "to", "subject", "message-id"},
+	Headers:               []string{"from", "to", "subject", "message-id", "date"},
 	BodyLength:            0,
 	QueryMethods:          []string{"dns/txt"},
 	AddSignatureTimestamp: true,