The correct answer is Tim Leffington.
Why would the killer write part of the off-line letter? If there was something incriminating in the email, he could have just erased it. Instead, he actually took the time to add something. Why?
Amy realized that the email was the one piece of evidence that set the time of death. The killer must have written this in order to set up a false time of death. And the only one of the suspects with an alibi for this time was Tim Leffington.
Tim, the secret admirer, had confronted Gretchen, who rejected him and caused him to go wild. After the murder, Tim needed an alibi. He saw Gretchen's unfinished email and sat down to add to it, mentioning the fight with Bess, a fight that had not yet occurred.
When Bess arrived, Tim instigated the fight, thus giving himself an alibi.