The quick answer
Clear the customer's Send Transactions Via Email preference, then edit the Sales Order form users actually select. Under Screen Fields → Communication: Messages, set To Be E-mailed's Check Box Default to Unchecked. Start a fresh Sales Order and inspect To Be E-mailed before saving.
The screens show a controlled example. The person and operating-company names are fictional example data. Use a customer and form whose email settings you are authorized to change.
Before you start
Use a role that can edit the customer and the intended custom Sales Order form. Identify the role and form used for new orders; a role-preferred or manually selected form can change which default applies. Clearing the customer's email preference can affect other applicable transactions for that customer, potentially including invoices and cash sales depending on the account's email setup. Confirm that scope before saving the customer.
01. Clear the customer preference
Open Lists → Relationships → Customers, edit the customer, and go to Preferences. Under Send Transactions Via, clear Email and save the customer. In the recorded example, the customer is Northstar Tools.
02. Check a new order
Start a new Sales Order for the same customer and role. Record the actual Custom Form shown on the order, including any role-preferred or manually selected form. Open Communication → Messages and inspect To Be E-mailed before saving. In this recorded sequence it is still selected after the customer preference change: clearing that preference does not override a checked form default. Leave this check order unsaved unless a real test order is authorized.
03. Change the selected form default
Open Customization → Forms → Transaction Forms and edit the exact custom Sales Order form identified on the new order. Go to Screen Fields → Communication: Messages, find To Be E-mailed, and set Check Box Default to Unchecked. Save the form and reopen it to confirm that the selection persisted. The setting applies to transactions that use this form, so inspect its role and subsidiary use before applying it broadly.
04. Verify a fresh Sales Order
Discard the first unsaved check order. Start another new Sales Order, explicitly select the intended form and customer, and return to Communication → Messages. To Be E-mailed should now be unchecked before saving. This is the success check shown in the recording. These settings do not bulk-clear existing orders, and scripts, workflows, integrations, or a manual Email or Save & Email action can still send messages. If you run an authorized send test, inspect the recipient and automation first; message history alone does not prove mailbox delivery or non-delivery.
Common questions
Why is To Be E-mailed still checked after I clear the customer preference?
The selected Sales Order form can still default it to checked. Confirm Custom Form on the new order, then inspect that form's Check Box Default for To Be E-mailed.
Where is the form's checkbox default?
Edit the selected form under Customization → Forms → Transaction Forms. In Screen Fields → Communication: Messages, find the To Be E-mailed row and set Check Box Default to Unchecked.
Why is the box still checked after I save the form?
Close the earlier check order and start a fresh one. Confirm it uses the form you changed. A different preferred form, manual form selection, or account automation can change the result.
Does this change existing orders or block all email?
No. Existing orders keep their values. A user can still check the box or use a manual email action, and workflows, scripts, or integrations can send separately.