Clarifications on MTA-STS Policy with CNAME Records
TechCommunity and The_Exchange_Team, In the article "Introducing MTA-STS for Exchange Online," there's a statement: The_Exchange_Team wrote: We do not support CNAMEs when MTA-STS is used. If a domain uses a CNAME and follows the MTA-STS RFC, that domain will fail our MTA-STS checks, and will not receive emails from us. I have several technical questions regarding this: CNAME Reference: Is the "domain uses a CNAME" part specifically referring to MX servers using CNAME records, or the mta-sts and _mta-sts DNS records? CNAME Usage: Many MTA-ST hosting services utilize CNAME records for MTA-STS configurations. Additionally, it appears Microsoft uses CNAMEs for serving its MTA-STS policies. What's the rationale? Record Type Implications: Is there a functional difference, from a Microsoft implementation standpoint, if an mta-sts record is served via an A/AAAA record vs. a CNAME record? Looking for technical clarifications on these points.