Everyone here told you that you have to use spf. But beaware that you need to put the directive -all and not the ~all. Just take care of this point as it's very

Everyone here told you that you have to use spf. But beaware that you need to put the directive -all and not the ~all. Just take care of this point as it's very important! Second please check all of your systems that your email password is entered atleast once. Maybe a system has a spyware or virus or something. Third change your email's password. Except you are 100% sure that the spammer is not using your MTA then you just use an spf with the -all directive!