When emails start landing in spam, almost everyone does the same first thing: rewrites the email. Swaps “free” for “complimentary,” deletes an exclamation point, ships the same campaign again — to the same spam folder.

Here’s the 2026 reality: modern filters are reputation-and-engagement models, not keyword lists. The words are a minor input. Who is sending matters far more than what is being said — which is good news, because “who is sending” is a checklist you can fix in an afternoon.

Run the diagnosis in this order. Stop at the first thing that’s broken; it’s probably the answer.

1. Authentication (5 minutes, fixes the most cases)

Since Google’s and Yahoo’s 2024 rules and Microsoft’s 2025 enforcement, mail that fails SPF, DKIM and DMARC doesn’t get judged on its merits — it gets pre-sorted into junk, or rejected outright with an error code.

Run your domain through the free Domain Health Checker: SPF, DKIM, DMARC and MX in one pass. The classic silent killers it catches:

SPF past the 10-lookup limit — permerror, every send fails
DKIM signed with the provider’s domain instead of yours — valid signature, zero alignment
No DMARC record at all — now a hard requirement for volume senders

2. Reputation: are you on a blocklist? (3 minutes)

Your domain or sending IP may already be on one of the DNS blocklists receivers consult. The free Blocklist Checker sweeps both across 13 live DNSBLs. If you’re listed, here’s the delisting playbook — fix the cause first, then request removal. Until then, content edits are rearranging deck chairs.

3. The complaint and bounce math (the slow killer)

Two thresholds rule everything:

0.3% spam complaints — Gmail’s stated limit. Past it, the algorithm isn’t filtering you, it’s policing you. Stay under 0.1%.
~2% bounces — above this, receivers conclude you don’t know who you’re mailing. Old lists rot fast; verify before sending and clean the file before every campaign.

Both numbers are ratios, which is why low-volume senders get surprised: at 1,000 sends, three complaints reaches the limit. Tight targeting isn’t a nice-to-have; it’s arithmetic.

4. Engagement: the filter that never announces itself

The hardest input to fake: do recipients open, read, reply — or delete unread? Providers in 2026 weight engagement aggressively. Sending to a list that ignores you teaches the filter, address by address, that you belong in spam. (Worse, much of the engagement you think you have is robots — judge segments by replies, never opens.) Prune the segments that never respond. Shrinking your list is often the single most effective deliverability move available.

5. Now the content (last, honestly)

Words can still hurt at the margins — mostly by pattern-matching with two decades of spam. Worth a pass:

The Spam Word Checker highlights filter-bait phrasing in place, with an honest density score — the problem is twelve red flags, never one “free.”
The Subject Line Tester runs the cold-email checks (length, caps, bait punctuation) with a Gmail-style preview.
The Email Size Checker weighs your HTML against Gmail’s 102KB clip — clipped emails hide your unsubscribe link, which circles back to complaints.
Image-heavy signatures and tracking-link soup read spammy; a clean HTML signature doesn’t.

The uncomfortable summary

“Spam words” survive as an explanation because they’re fixable in five minutes and nobody has to admit the list is bad. The real rank order is: authentication, reputation, list quality, engagement — then words.

Fix them in that order and placement usually recovers in two to four weeks of consistent, well-behaved sending. Reputation has memory; so does the fix.