The following are the options in the Roundcube config:
// Behavior if a received message requests a message delivery notification (read receipt)
// 0 = ask the user, 1 = send automatically, 2 = ignore (never send or ask)
// 3 = send automatically if sender is in addressbook, otherwise ask the user
// 4 = send automatically if sender is in addressbook, otherwise ignore
$config['mdn_requests'] = 0;
// Return receipt checkbox default state
$config['mdn_default'] = 0;