Well, Max--In a sentence--"I ignore many e-mails, but this one called for an e-ply." Or--"An original with signature is not required, an e-ply will do." Or--"I won't be near my fax tomorrow, please e-ply." Or eply. Differentiates between the initial and subsequent e-messages.
But don't use the word if you don't like it, I didn't mean to start an e-mission!