Amazon contacts customers via the submitted email with tracking details when this field is filled out.
Customers complain because they get confused with this Amazon email.
The product title in the email comes from Amazon listing and is different from the non-amazon order. Also, they don't understand why Amazon contact them even they never bought anything on Amazon.
It's a big mess and we discussed this with Amazon. Their solution is to just not fill out this optional email field. If we fill it out, they will automatically email all customers whenever an order is shipped.
Linnworks should set the email field as an optional setting when transferring orders to Amazon FBA Fulfillment.