Fix error name of ListenAndServe

[anaxita]

main.go lines 78-81

Change

	err_:= smtpd.ListenAndServe(listen, mailHandler, "mail2tg", "", debug)
	if err_ != nil {
		log.Fatal(err_.Error())
	}

To

        err = smtpd.ListenAndServe(listen, mailHandler, "mail2tg", "", debug)
	if err != nil {
		log.Fatal(err)
	}