Why is Civimail sending exactly 30 emails every cron run through Office365 SMTP Relay?
I'm not familiar with Office365 Relay, a quick search indicates it's for devices to send emails via Office365. It was a quick search so I may be wrong. 

Given that you are only able to send 30 emails at a time and that Office365 has a 30 emails/minute limit sounds like it is something to do with Office365 not a CiviCRM setting issue.

You may want to look at a SMTP service, here's a blog post that compares some services that work with CiviCRM.