Exchange Online now supports SMTP Strict Transport Security (MTA-STS), a mechanism to help defend SMTP communications between mail servers. Microsoft 365 tenants can decide if they want to enable MTA-STS for their domain by publishing a DNS record and an MTA-STS policy. You don't have to use MTA-STS, but it's a good idea to consider the option.
In a December 24 announcement, Microsoft says that the roll-out of DNSSEC and DANE support in Exchange Online will start in mid-January 2022. Because this is a big change for the Exchange Online infrastructure, Microsoft is using a phased deployment which won't complete until mid-May. Support for DNSSEC and DANE has been coming for a long time, but it's good that the extra security which these standards bring will be available to Exchange Online tenants.