MBP Spotlight: outlook.com

Who hosts what mail? MBP Spotlight
is the new series here at Spam Resource where I’ll share ranking data for the top mailbox providers (including B2B!) based on where they fall in the top ten million domains. Some entries might have more info, some entries might have less, depending on what information I know about them and what information I can find online.

To count rankings, I took a snapshot of MX records for the top ten million domains, and then rolled them up based on the domain name hosting mail for each domain. That shows me how many domains have their inbound email hosted by hosted by “*.google.com,” “*.outlook.com,” etc.

Number 2 in the rankings: outlook.com

Who is this? Microsoft

What are they? Microsoft Corporation is an American multinational corporation and technology company headquartered in Redmond, Washington.

How much mail do they host? The domain outlook.com hosts mail for more than 792,765 email domains.

There are a few different types of Microsoft outlook.comMX records: 
  • Any domain whose MX record ends in “olc.protection.outlook.com” is an “OutLook Consumer” domain. This means consumer webmail, aka Hotmail, Live.com or Outlook.com
  • Any domain whose MX record ends in “mail.protection.outlook.com” is usually a B2B customer of Microsoft’s O365 email service. (Microsoft also uses this hosting for some of its own corporate domains, for example azure.com.)
  • Any domain whose MX record ends in “mail.eo.outlook.com” is likely also an O365 customer whose inbound MX record points at an old Microsoft endpoint. I believe the implication is that mail to these domains is handled similarly to those hosted under “mail.protection.outlook.com.” Let me know if I’m wrong on that.
  • Domains whose MX record ends in “mx.microsoft” (yes, TLD of “microsoft”) are B2B customer domains where DANE is enabled.
  • There’s a whole other set of customers hosted under microsoft.com. At the moment, I don’t have any data on how these are handled differently.
I’m sure there’s more to this – we’re probably just barely scratching the surface here.

Here are just a few of the top companies using Microsoft to host their corporate mailboxes (as of April, 2024): Hilton, Shopdotcom, Digicert, Bristol Myers Squibb, SAS and T-Mobile.

I’ve written extensively about Microsoft deliverability challenges in the past, the different types of bounces and rejections you can receive when sending mail to Microsoft domains, and I’ve put together a Microsoft OLC deliverability troubleshooting guide.

Microsoft offers an ISP feedback loop, called the JMRP (Junk Mail Reporting Program). This covers consumer domains only (aka Outlook.com/Hotmail domains, the ones that have an MX pointing to *.olc.protection.outlook.com).

Ranking based on March 2024 data snapshot. Corrections, questions or updates? Contact me.
Post a Comment