Shared E-Mail Inbox Causing Duplicate Sales Orders To Be Input - Warning Inadequate

You can make it an error instead of a warning(on the Order Type page) but there isn’t an out-of-the-box way to make it a popup instead. 

If you go the customization route it looks like the business logic is implemented via a CustomerOrderNbrAttribute(could be different depending on your version) so you could extend that attribute and add a popup if the field has an error set and then override the attribute on the CustomerOrderNbr field with your custom one.

Below is a quick code sample

    public class SOOrderEntryExt: PXGraphExtension<SOOrderEntry> {
        public static bool IsActive() => true;

        [PXMergeAttributes(Method = MergeMethod.Merge)]
        [CustomerOrderNbrExt]
        public void _(Events.CacheAttached<SOOrder.customerOrderNbr> e) { }
    }

    public class CustomerOrderNbrExtAttribute: CustomerOrderNbrAttribute
    {
        public override void FieldVerifying(PXCache sender, PXFieldVerifyingEventArgs e)
        {
            base.FieldVerifying(sender, e);

            HandlePopup(sender, (SOOrder)e.Row);
        }

        public override void RowUpdated(PXCache sender, PXRowUpdatedEventArgs e)
        {
            base.RowUpdated(sender, e);

            HandlePopup(sender, (SOOrder)e.Row);
        }

        private void HandlePopup(PXCache cache, SOOrder order)
        {
            if (order == null) return;
            if (string.IsNullOrEmpty(order.CustomerOrderNbr)) return;

            var fieldState = GetFieldState(cache, order);

            RegisterPopupIfInError(fieldState, cache, order);
        }

        private PXFieldState GetFieldState(PXCache cache, SOOrder order)
        {
            return cache.GetStateExt<SOOrder.customerOrderNbr>(order) as PXFieldState;
        }

        private void RegisterPopupIfInError(PXFieldState fieldState, PXCache cache, SOOrder order)
        {
            if (fieldState?.Error != null)
            {
                PopupNoteManager.RegisterText(cache, order, nameof(SOOrder.customerOrderNbr), fieldState.Error);
            }
        }
    }