Is calling a staff member "selfish" harassment? A faculty member wrote to a staff member that "you are selfish." The staff member wrote back to the faculty member asking for an explanation. The faculty member did not respond and wrote: "I would not respond to you any further." Is this harassment?
It's brusque and ill-advised, but it's not harassment per se. 

Harassment is the creation of a hostile work environment through pressure or intimidation. A one-off heated email and the use of a single adjective to describe someone in a negative light is certainly not sufficient of proving anything more than the person who wrote the email probably should have waited a while before hitting the "send" button. A sustained campaign of emails and a larger pattern of behavior would be needed to make a believable harassment argument.

There's no reason for this to be escalated into a larger situation that will ultimately accomplish very little except inflaming tensions further.