your friendly neighborhood deliverability guide since 2001
Sending mail over IPv6? Authenticate!
Josh over at Word to the Wise explains that as Microsoft brings live IPv6 support for its Office365/Exchange Online Protection email platforms, they're mandating that all mail sent over IPv6 must authenticate with either Sender Policy Framework (SPF) or DomainKeys Identified Mail (DKIM). Unauthenticated mail will be rejected. Read more about it here.