Why your cold emails go to spam in Gmail (but not in Outlook)

You send a clean, authenticated, short cold email. It lands in the inbox at Outlook and goes to spam at Gmail. That is exactly what happened to us in mid-September 2026. We measured, ruled out the false leads one by one, and found the real cause. Here is what we learned about email deliverability, with the numbers.

What exactly did we observe?

To find out where our emails land, we send test emails to mailboxes we control at Google and Microsoft, then check whether they reach the inbox or the spam folder.

In mid-September we ran 53 tests, across three recent sending domains, six mailboxes and two completely different texts. The result left no room for doubt:

The same emails, sent from the same mailboxes, were accepted without reservation by one and rejected outright by the other. A result this clear-cut has one advantage: it lets you rule causes out.

Is it an authentication problem?

It is the first thing to check, and it was our first guess. Three DNS records tell mailbox providers that you really are the sender:

• SPF lists the servers allowed to send for your domain;
• DKIM signs each email to prove it was not altered;
• DMARC says what to do with an email that fails the first two.

Across our 53 tests, SPF, DKIM and DMARC passed everywhere, at Google and Microsoft alike. Authentication was correct. It remains essential, but it was not enough.

Is it the email copy?

Second guess: "spammy" words, a message too short or too salesy. We tested two very different texts:

• a three-line message, deliberately terse;
• a 90-word message, concrete, explaining a real offer.

At Google, nothing moved: both ended up in spam. At Microsoft, both reached the inbox. The copy was not the cause.

So what makes the difference at Gmail?

Once authentication and copy are ruled out, what remains is sender reputation: that of the domain and that of the IP address of the server that sends. Google judges it by its own criteria, and clearly not the way Microsoft does.

Our three domains were recent, and they sent from our former setup: a self-hosted mail server. To separate domain from IP, we built a new setup with a mainstream email host: 15 mailboxes on 3 new domains.

On the same day, with the same text, we compared the two:

The only difference between the two columns was the sending IP address. That is what Google was penalising.

What do we check every week now?

A reputation is never won for good. So we turned this test into a routine:

One rule matters more than the others: a test that could not conclude never counts as a pass. If an answer is missing, the mailbox is treated as unknown and it waits. It is less comfortable, but it is the only way not to send blind.

Modest volumes

Our mailboxes send about 5 cold emails a day each. Ramping up volume too fast is one of the surest ways to damage a new reputation. Several mailboxes sending a little beat one mailbox sending a lot.

How can you tell if your emails go to spam?

No need to wait for prospects not to reply. You can measure it yourself:

• create one or two test mailboxes at Gmail and at Outlook;
• send them exactly the cold email you are about to send, from the same mailbox;
• note where it lands: inbox, Promotions tab, or spam;
• repeat every week, and for every new domain or mailbox.

An invalid address hurts your reputation too: every bounce counts against you. Verify addresses before writing, as we explain in our guide to B2B prospecting.

What should you do if Gmail sends you to spam?

In order, from cheapest to most radical:

• Check authentication: SPF, DKIM and DMARC must pass. That is the baseline.
• Test two different texts: if both end up in spam, it is not the copy.
• Compare Gmail and Outlook: a clear gap points to your reputation at Google.
• Change your sending server if the gap persists: that is what unblocked everything for us.
• Keep volumes low and use a domain dedicated to prospecting, to protect your main domain.

Deliverability does not replace a good message or a good moment to write. It only decides whether your message is seen. For the substance, see our guide to sales prospecting.