Ibn 'Abbas said, "I think that answering a letter is a duty, just like returning the greeting."  

عَنِ ابْنِ عَبَّاسٍ قَالَ إِنِّي لَأَرَى لِجَوَابِ الْكِتَابِ حَقًّا كَرَدِّ السَّلامِ