Email was invented by Ray Tomlinson in 1971.
It was improved in 1972 to include multiple users.
On a related note, the world's first integrated email system was invented by Dr. V. A. Shiva Ayyadurai aka Dr. E-Mail from MIT. He designed the first email system in 1980. He received the first copyright for email (TXu 111-775) from the United States Copyright Office in 1982. This was before patents were issued for software which was treated similar to music, art or literary work.