thE EDitor At onCE sEnt thE journAlism A FAx in...

in the fax，一般是紧跟着的那个名词。顺过来就是 He had at last been allowed to send a fax. In the fax he informed the editor that he had been...

which引导的定语从句，which 在句中代指fax,editor 是在fax中的，所以有个从属关系，这里用介词in表示从属关系，he informed the editor in the fax