I hate answering questions with "That depends," but it does in this case.
If your customer is using your bank's Internet Banking service to send you the stop order, you could readily adopt and disclose a policy that says you won't require a manually signed stop order to ratify the electronic request (since the customer had to log in to a presumably secure server first).
If the customer is sending you a public server e-mail without some form of sender authentication, you might want to continue requiring a manually signed stop order.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8