Atticus was leaving the post office when Mr. Ewell approached him, first threatening him and then challenging him to a fight. Atticus' response when Ewell jeered "too proud to fight, you nigger-lovin bastard" was a witty, "No, too old" which was effective for diffusing the situation for the moment, but did not give Ewell any satisfaction and might have contributed to Ewells hatred for the Finches and the later attack on Jem and Scout.
It might have been wiser for Atticus to have taken a punch from Ewell, so that Ewell would feel that he had his vengeance. In any case, Atticus probably should have taken the threat more seriously and taken precautions for himself and his family. I wonder that if he had taken some stronger stand against Ewell if he would have backed off. He backed of of harassing Helen Robinson after Linc Deas made it clear that he would have him arrested for harrassment or assault if he kept bothering her.