There is no such thing as your inbox rate. Split every metric by recipient MX.

There is no such thing as your inbox placement rate. There is your placement at Google, your placement at Microsoft, and your placement at whatever else is in your list. Averaging them produces a number that describes nothing and hides the problem.

Two campaigns can both report "82% inbox" while one is fine and the other is quietly dead in half its market.

The receivers do not filter the same way

This is the mechanism, and it is why the average misleads.

Google leans heavily on engagement. Opens, replies, moves out of spam, deletes without reading, and complaints, aggregated per sending domain over a rolling window. Reputation moves relatively fast in both directions. If people engage, you can send more. If they stop, you fall off, even with perfect authentication. Google Postmaster Tools exists precisely because this is measurable and they will show you some of it.

Microsoft leans much harder on IP and sending-pattern reputation. Consumer Outlook and Microsoft 365 filtering are not identical, but both weight the sending IP's history more than Google does, and both are less forgiving about sudden volume changes. Recovery is slower and less transparent. There is no Postmaster Tools equivalent that tells you your standing, which is why Microsoft problems tend to be discovered late, usually as unexplained silence from a whole segment.

The practical consequence: a shared-IP sending setup can be fine at Google and terrible at Microsoft at the same moment, because your neighbours on that IP hurt you at one and are invisible at the other.

Everything else in most B2B lists is smaller and messier. Custom-domain Google Workspace and Microsoft 365 tenants inherit the base filtering plus whatever the admin added. A tenant with an aggressive third-party gateway in front of it can reject you for reasons no amount of warmup touches.

Split every metric by recipient MX

The mechanic is simple. Resolve the MX record for each recipient domain, bucket it into Google, Microsoft, or other, and store it on the contact. Then split everything you already measure by that bucket:

• Bounce rate
• Reply rate
• Complaint rate where you can see it
• Seed test placement

You do not need a new tool for this. It is a DNS lookup per domain, cached, and most sequencers will let you store the result as a custom field.

What the split tells you

Bad at Microsoft, fine at Google. Look at IP reputation and sending pattern. Are you on a shared pool? How fast did volume ramp? Microsoft punishes sudden increases harder. This is also the case where a dedicated IP or a different mailbox provider genuinely helps, as opposed to the many cases where people buy one for no reason.

Bad at Google, fine at Microsoft. Look at engagement and complaints. Google is telling you that recipients do not want this. That is a targeting or offer problem wearing a deliverability costume, and no infrastructure change fixes it. Open Postmaster Tools and check spam rate first.

Bad at both. Authentication, list quality, or content. Start with bounce rate and SPF/DKIM/DMARC, because a shared cause is usually a boring one.

Bad at one specific custom domain and nowhere else. That is a gateway or an admin rule, not your reputation. Chasing it globally is wasted effort.

The knock-on effects worth knowing

Your warmup pool is probably lopsided. Most warmup networks are heavily Google, because Google mailboxes are cheap and easy to spin up. If your warmup traffic is 90% Google and your prospect list is 40% Microsoft, you have been warming for a receiving side you barely send to. Check what your warmup provider's pool actually looks like, and ask specifically for the Microsoft share.

Your list mix determines your risk. A list heavy in enterprise and regulated industries skews Microsoft, so a Microsoft reputation problem takes out most of your pipeline. A list heavy in startups and tech skews Google. Same infrastructure, completely different exposure.

Segment sending, not just measurement. Once the field exists, you can throttle the receiver you are weakest at while you fix it, instead of pausing everything. Slower on Microsoft while reputation recovers, normal at Google.

The version to run this week

1. Resolve MX for every domain in your list. Bucket into Google, Microsoft, other.
2. Split your last 30 days of bounces and replies by bucket.
3. If the two buckets differ by more than a few points, you have been debugging an average.
4. Fix per receiving side, using the mechanism above to pick where to look.

The stated caveat: MX bucketing tells you who filters the mail, not the tenant's local rules, and it will not explain a single company blocking you. It is a first cut that turns one useless number into two useful ones.

Post your split in the Tuesday clinic if you run it. Bucket sizes, bounce rate per bucket, reply rate per bucket. A few of those side by side would be more useful than another thread about warmup length.