About a year ago, I wrote an article entitled DKIM and DMARC in Office 365 where I explored what DomainKeys Identified Mail (DKIM) and Domain-based Message Authentication, Reporting & Conformance (DMARC) are, and how exactly they work with Exchange Online in Office 365.
At the time of writing that article, Office 365 only supported inbound validation of DKIM over IPv4 and IPv6. Outbound DKIM signing was not yet available but was in the roadmap. Well, it is here now!
To continue reading about DKIM signing in Office 365, please check this article at MSExchange.org.