I just discovered to my chagrin that my phone was sending read receipts to any sender who requested one, without asking me if I wanted to send one. This could be one of the causes that my spam has increased greatly since I got a new Android phone to sadly replace my favorite Windows Phone.
In Outlook on the Web, under mail options / general / mobile devices, there is a checkbox that says "Don't send read receipts for messages read on devices that use Exchange ActiveSync" I'm not sure if I unchecked this by accident at some point since I started using O365, or if the default is that it's unchecked. It has to be CHECKED in order to stop the read receipts from being sent without notice. (See below:)
Exchange Online - Read Receipts for Mobile DevicesIf, in fact, the default is that it's left unchecked, I think this is a bad idea. At the least, I think it should be a required prompt when a new mobile device is added and set up to sync with an account.