We're looking at Exchange Online as well, and also have a Barracuda mail archiver. IIRC, the Barracuda has to go, and there's no two ways about it.

Exchange Online users soon will be getting some security enhancements that will enforce the use of Transport Layer Security (TLS) encryption for e-mails and ward off so-called 'man-in-the-middle ...

Update 2: Exchange Online mailflow has been restored. After extended monitoring we confirmed that the queued emails have been processed as expected, and mail flow impact for Exchange Online in ...

Microsoft provides more details on implementing Inbound SMTP DANE with DNSSEC for Exchange Online mail flow in this tech community post. 8 Common Threats in 2025.