For only basic email traffic needs like scan to email, just setup a connector in Office 365.
Exchange Online has various rate limits though. So if you think you will be even close to them, use postfix locally to send out and just add your local WAN IP to your SPF record.
Why wouldn't your local relay still relay everything through Hosted Exchange in O365? or would that be rate limited as well?
Exchange Online has limits. it does not matter from where the email originate. The software was actually setup to authenticate and send email directly with and Exchange Online account during testing.
This quickly hit the rate limits as the client was sending out large amounts of price notifications to users that has signed up for such.
So this site had a local CentOS 7 box setup and Postfix was configured to send mail out. The SPF record was updated and then a connector was also setup to make sure nothing was blocked for no reason.