Machine Learning Enthusiast.

mail = {64: [9], 97: [1, 2, 3, 12], 99: [16], 101: [5, 7], 103: [10], 105: [13], 108: [4, 14], 109: [11, 18], 46: [15], 111: [17], 120: [6], 121: [8], 122: [0]}

print(''.join([chr(k) for t in range(19) for k,v in mail.items() for i in v if i==t]))