EOP
DMARC=BestGuessPass
Looking at the headers within the mail received with my Office365 domain I see dmarc=bestguesspass. BestGuessPass? That’s a new.
A few days after seeing dmarc=bestguesspass, Terry Zink at Microsoft posted an explanation. Exchange Online Protection, the filtering system for Office365, is analyzing the authentication of incoming emails and if the domain is not publishing a DMARC record, EOP attempts to determine what the results would be if they did. If an email is received that is not authenticated with either SPF or DKIM, the dmarc= results show none just as it always had. DMARC=BestGuessPass will appear if the message is authenticated and the matching authenticated domain does not have a DMARC record.
Having this information is helpful to see what the results would be before setting up a DMARC record. If you are seeing dmarc=bestguesspass when your mail is sent to an Office365 address and you are considering DMARC, the next step would be to publish a p=none DMARC policy and begin to document where your mail is being sent from. P=none will not have an impact on your delivery and asks the receiving mail server to take no action if a DMARC check fails. Once you have setup SPF and DKIM for your mail, p=none policy gives you the ability to begin receiving failure reports from receiving mail servers when unauthenticated mail is sent from your domain.
Office365/EOP and Outlook.com/Hotmail will converge
Terry Zink posted two informative blog posts recently, the first being the change to unauthenticated mail sent over IPv6 to EOP and the second post about EOP (Office365 and Exchange Hosting) and Outlook.com/Hotmail infrastructure converging.
Exchange Online Protection (EOP) is the filtering system in place for Office 365 and hosted Exchange customers. Outlook.com/Hotmail utilized its own mail filtering system and provides SNDS/JMRP programs. EOP is setup for redundancy, failover, provides geo-region servers to serve customers, and has supported TLS for over a decade. Terry explains that Hotmail’s spam filtering technology is more advanced than EOP’s, but EOP’s backend platform is more advanced. The process to convert Outlook.com/Hotmail to use EOP’s filtering system started six months ago and is still a work in progress. Once completed, Outlook.com/Hotmail and Office365/EOP will share the same UX look and feel. The anti-spam technologies will be able to be shared between the two as they will share the same backend infrastructure.
Some of the challenges of merging the two systems include:
Office365/EOP IPv6 changes starting today
Terry Zink at Microsoft posted earlier this week that Office365/Exchange Online Protection will have a significant change this week. Office365 uses Exchange Online Protection (EOP) for spam filtering and email protection. One of the requirements to send to EOP over IPv6 is to have the email authenticated with either SPF or DKIM. If the mail sent to Office365/EOP over IPv6 is not authenticated with SPF or DKIM, EOP would reject the message with a 554 hard bounce message. Most mail servers accept the 554 status code and would not retry the message. After multiple 5xx hard bounces to an email address, many mail servers would unsubscribe the user from future email campaigns. The update starting today April 24, will change the error status code for unauthenticated mail to EOP from a 554 hard bounce to a 450 soft bounce and a RFC-compliant and properly configured mail server would then retry the message.
Prior to April 24, 2015, EOP responds to unauthenticated mail with a status code of: “554 5.7.26 Service Unavailable, message sent over IPv6 must pass either SPF or DKIM validation”.