A message can be perfectly authenticated, correctly coded, and still end up in junk. Authentication decides whether the message is admissible; reputation decides whether it is welcome. Here are the six causes of spam placement that actually occur at small volumes, in the order they appear.
1. A list built without explicit consent
This is the dominant cause, and the only one no setting can fix. A list imported from an address book, purchased, or built with pre-ticked boxes mechanically produces complaints and dead addresses. Providers measure exactly that.
| Indicator | Accepted threshold | Effect of exceeding it |
|---|---|---|
| Complaint rate (“report as spam”) | < 0.1% — alert from 0.3% | Junk placement, then blocking |
| Invalid addresses (hard bounces) | < 2% | Rapid degradation of sending reputation |
| Spam traps | None tolerated | Domain or IP blocklisting |
Double opt-in costs subscriptions in the short term and gains them in the medium term, because it eliminates typos and addresses entered by third parties.
A list of one thousand consented addresses delivers better than a list of ten thousand collected ones. That is not a moral position, it is reputation arithmetic.
2. No compliant unsubscribe mechanism
Since 2024, bulk senders must offer one-click unsubscription compliant with RFC 8058, processed within two days. In practice, two headers must accompany the send:
List-Unsubscribe: <https://example.com/unsubscribe?t=TOKEN>, <mailto:[email protected]>
List-Unsubscribe-Post: List-Unsubscribe=One-ClickThe reasoning is counter-intuitive but sound: easy unsubscription prevents a spam report. The first act is reputation-neutral, the second degrades it lastingly.
3. A sending domain with no history
A new domain, or a dedicated IP address never used, has no reputation. Sending several thousand messages straight away from such an origin triggers anti-spam protections.
Progressive warm-up is the standard answer: a few dozen sends in the first days, prioritising the most engaged contacts, then progressive doubling over two to four weeks. At small volumes, a well-managed shared IP from the sending platform generally performs better than an under-used dedicated IP.
news.example.com) from transactional ones (mail.example.com) protects the latter. A campaign that generates complaints then does not drag order confirmations down with it.4. Weak engagement, sustained
Modern filters weigh recipient behaviour heavily: opens, replies, filing, deletion without reading. Continuing to write to contacts who have not opened for a year does not merely waste sends — it degrades delivery for everyone else.
Periodic cleaning is a deliverability action, not an accounting one:
- Segment inactives at six or twelve months depending on send frequency.
- Attempt a single, short re-engagement campaign with an explicit exit.
- Remove the non-reactivated from the main list.
5. Content that resembles what filters discard
Lists of “forbidden words” are largely folklore: current filters assess a bundle of signals. Some remain penalising and easy to avoid:
- A message made of one large image with no usable text.
- No plain-text version.
- Generic URL shorteners, heavily used by malicious campaigns.
- A link-tracking domain different from the sending domain and unauthenticated.
- A mismatch between subject line and content, which produces immediate deletions.
6. Infrastructure that contradicts the message
A final point, often neglected on small sites: the subscription form and the site itself are part of the signal. A site served without HTTPS, an unsubscribe page returning an error, an expired tracking domain or a bouncing reply address all degrade the whole. Technical consistency between the message and the infrastructure carrying it is checked.
Diagnosing in the right order
- Read the DMARC reports: they reveal unauthenticated sends made in your name.
- Consult the postmaster tools of the major providers, which expose complaint rate and domain reputation.
- Check public blocklists for the domain and the sending IP address.
- Segment by provider: a problem limited to one operator points to reputation, a general problem to authentication or content.
What to take away
Deliverability is not a setting but a regime. The three levers that weigh most are, in order: how the list was built, how easy it is to leave, and how consistent engagement is. Content and infrastructure come afterwards — they rarely explain junk placement on their own.
On my own sends, the one measure that produced an immediate and lasting effect was removing contacts inactive for more than a year. I lost a third of a list and gained delivery on the remaining two thirds. It is the most counter-intuitive decision in the discipline, and the most profitable. — Simon Janvier
