Experiment 001, deliverability
One header sent our emails to spam in Outlook.
Our reply rate on Microsoft was way lower. Before touching the copy, we checked the infrastructure first: do our emails even land in the inbox?
Gmail said inbox. Outlook said spam.
We sent a campaign email to Gmail: inbox. The same email to Outlook: spam. Same email, so it was not the copy. Something in it, Microsoft did not like, and that could explain the lower reply rate.
The link we did not put there
We opened the raw source of that email. There was a link in it. Everyone knows you do not put a link in a cold email, and we had not. It was in the unsubscribe header.
Every email from the sending platform carries that header when the option is on, and ours was on. We had never really thought about it. The link is not on our domain. It sits on a shared domain the platform uses for every customer, and it was the only link in the whole email.
The paired test
Same day, same inbox:
| Header | Result | |
|---|---|---|
| Campaign email to Gmail | On | Inbox |
| Same email to Outlook | On | Spam |
| Same email to Outlook | Off | Inbox |
| Different copy to Outlook | On | Spam |
| Different sending domain to Outlook | On | Spam |
SPF, DKIM and DMARC passed on every one. Every time, it was the header.
The clue we had heard before
A well known cold email operator once said his team even spins the unsubscribe language. Language, not link: you cannot spin a link. So he was not using the header either.
What we do now
Header off. The opt out is one plain line at the bottom of the email: if this is not relevant, just reply “no” and we stop. No link, no unsubscribe page.
To be clear: that is four emails into one inbox. It is not a proper study. On a different list, a campaign with the header on still got replies from Microsoft users. Test it on your own sends before you change anything.
Try this
If you have trouble landing in Microsoft inboxes, send a real campaign email to your own Outlook. Open the raw source and search for List-Unsubscribe. If that link is not on your own domain, you have the same question we had.