Hello,
This has been asked and answered previously, but the latest post I can find about it is a couple of years old, here:
To briefly summarize the situation, I have a customer who is hoping to change the default behavior of his iPhone's Outlook application. Specifically, when he clicks reply on any thread (regardless of if it's one-to-one or one-to-many) he'd like the default to be a direct-reply, not a reply-all.
My understanding at this point is that this cannot be changed, and the default option will be reply-all for any messages with more than one recipient. However, as I stated previously it's been a couple of years and I thought I'd see if see inexorable march of technological innovation had changed that state of affairs.
Many thanks in advance for your help!