There is a config option for how to handle the requests:
// 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 my contacts, otherwise ask the user
// 4 = send automatically if sender is in my contacts, otherwise ignore
// 5 = send automatically if sender is a trusted sender, otherwise ask the user
// 6 = send automatically if sender is a trusted sender, otherwise ignore
$config['mdn_requests'] = 0;