Royal Mail’s share price slumped on Tuesday to its lowest point since privatisation after declining junk mail and business uncertainty meant it delivered fewer letters. Addressed letter volumes, excluding the impact of elections, fell by 8% year on year in the nine months to 23 December, while letter revenues fell by 6%. Companies have been forced to scale back their mass mail marketing campaigns because of tighter EU privacy rules introduced under the General Data Protection Regulation. Meanwhile, businesses continue…