The error message is clear
Access denied, traffic
not accepted from this IP
So they aren't allowing your IP through. The give a link: http://go.microsoft.com/fwlink/?LinkId=526653
After visiting the link, look up the error code "5.7.708" which states:
5.7.700-749
Access denied, tenant has exceeded threshold
The majority of traffic from this tenant has been detected as suspicious and has resulted in a ban on sending ability for the tenant.
Ensure that any compromises or open relays have been resolved, and then contact support through your regular channel.
So, the way to solve it is to make sure you don't have an open relay on your mail server. I know you're using office as your mail server for this app, but that doesn't mean you don't have an insecure mail server installed on your host that you aren't using. You'll have to contact their support for more info. No one here can clear that up for you as it's MS that's blocking the IP address of your server. Maybe that IP has a bad "reputation", or is in some RBL. You can check for that here: https://mxtoolbox.com/blacklists.aspx and enter your site's IP address.