When mail stops landing, most people start rewriting subject lines. That is almost always the wrong end of the problem. Here are the real causes, in the order you should check them — the first three account for the overwhelming majority of cases.
No SPF, no DKIM, no DMARC, or one of them silently failing after a DNS change. This is the most common cause by a wide margin, and it is entirely binary: either the records verify or they do not. Check your provider dashboard first, before touching anything else. Full explanation here.
A new domain sending at volume looks like a spammer. A domain that previously sent badly carries that history. Neither is fixed by better writing. New domains need warming up; genuinely burnt domains are usually cheaper to replace than to rehabilitate.
Too much, too fast, or in bursts. A hundred messages in ten minutes is a machine signature no content can disguise. Safe volumes here.
Dead addresses produce bounces, and bounce rate is a direct reputation input. Scraped lists that were never verified are the usual culprit. Worse: some old addresses become spam traps — dormant addresses specifically used to catch senders who did not verify their lists.
This is the one nobody controls directly and everyone underestimates. If people mark your mail as spam, delete it unread, or never reply, providers learn that mail from you is unwanted. Conversely, replies are the strongest positive signal available.
Which leads somewhere uncomfortable but true: badly targeted outreach is a deliverability problem, not just a conversion problem. Writing to people with no plausible reason to care generates the exact behaviour that teaches providers to filter you.
Send test messages to addresses you control across several major providers and check where they land. Watch your reply rate over time — a sudden drop with no other change usually means placement changed. And use the free seed-testing tools that show inbox placement across providers, which is more reliable than guessing from silence.
Not automatically, and the folklore around word lists is mostly outdated. Modern filters weigh many signals together, and authentication and reputation dominate. That said, stacking a lot of promotional language in one short message contributes alongside everything else — it is a small factor, not the main one.
It can do lasting damage, especially on a young domain with little positive history to offset it. Recovery means stopping, fixing the cause, and rebuilding slowly over weeks. This is precisely the argument for a separate sending domain — so that the domain at risk is never the one carrying your invoices.
It depends who else is on it. Shared IP pools mean you inherit an average of everyone using them — good if the platform polices its users strictly, bad if it does not. Sending from your own domain and infrastructure means your reputation is entirely yours, which is riskier if you are careless and better if you are not.
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