Believe it or not, disabling Read Receipts in Outlook does not disable this feature from your mobile device. In fact, Send Read Receipts is enabled out of the box, and it has to be disabled via OWA. Big thanks to Gostev from Veeam for pointing this out!

Disabling it is easy, and I can’t think of many scenarios in which someone would NOT want to disable this. It can be disabled by logging into OWA from a computer (or a mobile browser that will disable the mobile view) and go to the following:
Settings -> General -> Mobile Devices -> and make sure to check the “Don’t send read receipts for messages read on devices that use Exchange Active Sync” checkbox.