Solution:
Open the file: root\m1\osc\settings\customers\customers_oscommerce.php
In the open file, locate the line:
$vtiger_data_map['emailoptout'] = '0';
replace the mentioned line with:
$vtiger_data_map['emailoptout'] = ($store_data_map['customers_newsletter'] == '' || $store_data_map['customers_newsletter'] == '0')?'1':'0';
Save changes and close the file. Run synchronization once again.
