Why rDNS is defunct for 365 users
Have you ever used a warmup tool, run delivery tests or tools like MX Toolbox and it says you need to set up reverse DNS (rDNS)? And then scratch your head for hours Googling it to see what the problem is? Have you stare into the abyss of DNS records and Microsoft’s documentation, getting nowhere? These rDNS issues aren’t necesssarily issues at all.
TL;DR: No, You Can’t Set rDNS in 365
(And You Don’t Need To)
Microsoft 365 does not let you set your own rDNS records. Not because they forgot. Not because you missed a checkbox. But because you don’t own the IPs your email comes from. Microsoft does. And they like to share. A lot.
That means your beautifully crafted newsletter or invoice is flying out into the world on shared, dynamic IPs that Microsoft manages. Those IPs resolve to something like:
mail-co1nam10olkn204.outbound.protection.outlook.com
Catchy, right? That’s the PTR (reverse DNS) record. It’s valid, it works, and unless you’re sending from an actual dumpster fire, it’s probably fine.
Why You Can’t Set Your Own rDNS
Okay, nerd hat on for a second. Here’s the why:
- You don’t own the IP. rDNS (a.k.a. PTR record) lives in the DNS zone of the IP address. Only the IP owner can edit it. That’s Microsoft.
- Microsoft uses shared and dynamic IPs. Your email could go out on IP A today, and IP B tomorrow, and IP Y on Thursdays after 3pm. You get no say in that.
So, even if Microsoft gave you access to set a PTR record, you’d be playing a DNS version of whack-a-mole.
But I Need rDNS to Pass Spam Filters!
(Do You Though?)
rDNS used to be the cool kid on the block. A way to say “Hey, I’m legit.” Now? It’s more like bringing a fax machine to a Zoom call. Respectable, but… maybe not essential.
Modern email deliverability is ruled by the SPF–DKIM–DMARC triad. Here’s the real MVPs:
- SPF: Says which servers can send mail for your domain.
- DKIM: Slaps a cryptographic signature on your messages.
- DMARC: Tells inboxes how to treat unauthenticated messages.
If those three are tight, you’re good in 99% of cases. rDNS is a minor signal at best. Google, Microsoft, Apple… they’re all looking at domain reputation, not reverse DNS pixie dust.
But What If Someone Demands rDNS?
Well, they’re either:
- Running a mail server from 2003 with a trust complex.
- Misunderstanding how cloud email works.
- Just being difficult. (Hi, procurement departments.)
If you’re getting blocked because a recipient’s server insists on a domain-matching PTR record, you’ve got three options:
- Reach out and explain you’re on Microsoft 365, and rDNS is centrally managed.
- Ask them to whitelist your domain or relax their filters.
- Use a third-party relay like SendGrid, Mailgun, or Postmark, which lets you send from a fixed IP with custom rDNS but now we’re adding cost and complexity.
Don’t Sweat the PTR Stuff
You can’t set rDNS in Microsoft 365, and that’s okay. Focus your energy where it counts:
✅ SPF, DKIM, and DMARC
✅ Consistent branding
✅ Clean mailing lists
✅ Not listening to every bounceback panic email
Need help checking your DNS setup or want to see if a smart host is right for you? Get in touch. Or, better yet, let’s run your domain through some tools and make sure your email house is in order.
Still, like, “huh”?
If all this makes your eyes glaze over faster than a Monday morning Zoom call, you’re not alone. Email deliverability is messy, nuanced, and often full of conflicting advice. That’s why we do what we do. At Quinset, we live and breathe this stuff — from diagnosing weird bouncebacks to architecting bulletproof DNS setups for modern platforms like Microsoft 365. Whether you’re troubleshooting a sticky rDNS issue or just want someone to sanity-check your email configuration, we’re here to help make sure your messages land where they’re supposed to: the inbox.
