925 cold emails from Microsoft 365, zero human replies

The numbers

We run outbound for our clients and for ourselves, from 22 sending domains. Five of them run on Microsoft 365, seventeen on Google Workspace. Same sequences, same prospect lists, same sending windows. Only the mailbox provider changes.

Method: 3 Aug to 9 Sep 2026. Each prospect counted once, in the week of their first email. Replies counted as distinct people, not messages. Our own test inboxes excluded. At the Google rate we would have expected about 8 human replies from the Microsoft side. We got zero.

What we ruled out

Our first reflex was the usual checklist. Every item came back clean, checked in DNS on 9 September.

So fixing the config would not have fixed anything. There was nothing broken to fix.

Your deliverability scores are measuring something else

Two tools told us everything was fine. Neither was measuring where the email actually lands.

The warm-up score is a closed loop between inboxes of the same tool's users. It proves your inbox can talk to other warm-up inboxes. It says nothing about a real prospect's Outlook.

mail-tester gave 8.3/10 to an email that Outlook put straight into junk. We tested it on 8 September: two identical emails, same minute, same recipient. One from a clean domain landed in the inbox. One from a domain listed on SURBL landed in junk. Both scored the same.

If you want to know where your email lands, send it to a real inbox at the provider you target and look. It takes five minutes and beats every score.

Théo Pascard, co-founder, Enjyn

The free placement test nobody uses

An out-of-office reply only fires when your email reaches the inbox. It does not fire from the junk folder. So the auto-reply rate measures placement, and nothing else: it does not care about your subject line, your copy or your targeting.

It also arrives in hours, not weeks, and it is about twice as frequent as human replies. On our data it separated the two providers from the first week, while human replies were still in the noise: 1.46% on Google, 0.22% on Microsoft (p < 0.001).

Limit: an auto-reply still fires from Gmail's Promotions tab. It tells inbox from junk, not Primary from Promotions.

What we still don't know

Here is the honest part. We route email so that Microsoft inboxes write to Microsoft recipients and Google to Google. That is standard practice, and it means 895 of our 925 Microsoft sends went to Outlook recipients. Only 2 went from Google to Outlook.

So two stories fit the data equally well. Either our Microsoft 365 inboxes are the problem. Or Outlook filters cold email hard, whoever sends it. From this data alone, we can't tell them apart.

We are testing it now: 398 Outlook prospects reached from Microsoft inboxes, 399 from Google inboxes, same sequence, same list, split at random. Results are due around 23 September. We will add them here, whichever way they go.

One thing that could bias it: the Microsoft inboxes also serve other campaigns and send about 1.7 times more per day than the Google ones. Load affects deliverability too. If the Google side wins, part of the gap may be load, not provider.

What we changed