Blog
Sourced notes, honest conclusions.
Researched, evidence-led writing on deliverability, email infrastructure, servers, GPUs, networks, and DDoS — each piece dated, sourced, and written to include the honest counter-case rather than to sell. When the right answer is a smaller plan, a different tool, or a competitor, we say so here too.
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Spam-complaint rate: the 0.1% / 0.3% line, how it's measured, and how to lower it
Gmail, Yahoo, and Microsoft all filter senders whose spam-complaint rate crosses 0.3%, with 0.1% as the real target. The catch is in the denominator and the fact that one bad send can trigger it. How to measure it and how to bring it down.
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Dedicated IP warm-up: the 4-8 week ramp, and whether you need one at all
A new dedicated IP has no reputation, so sending at full volume gets it throttled or blocked. The honest version: most senders should stay on a shared IP. For those who do need one, the segmentation rule, the daily ramp, and the metric gates.
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Outlook 550 5.7.515: Microsoft's bulk-sender rules and how to fix the rejection
Since May 2025, Outlook.com rejects bulk mail that fails authentication with a permanent 550 5.7.515 error. What triggers it, why the message is gone rather than in junk, the exact requirements, and how to diagnose which check failed.
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BIMI, VMC and CMC: how to put a verified logo (and the blue check) in the inbox
BIMI shows your brand logo next to authenticated mail — but only if DMARC is at enforcement first. The real prerequisites, the VMC-versus-CMC decision, what the Gmail blue checkmark requires, and an honest read on whether it is worth it.
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SPF: too many DNS lookups — the 10-lookup limit, PermError, and how to fix it
SPF is capped at 10 DNS lookups. Cross it and your record returns PermError — failing authentication for every message you send, silently. What counts, why flattening is a last resort, and why subdomain delegation is the honest fix.
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One-click unsubscribe (RFC 8058): the List-Unsubscribe-Post header, done right
Gmail and Yahoo's most under-implemented bulk-sender rule is not the footer link — it's the List-Unsubscribe-Post header. The exact headers, the POST endpoint, the five mistakes that quietly break it, and how to verify it for 2026.
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DMARC from p=none to p=reject: a safe migration guide for 2026
Moving DMARC to enforcement stops domain spoofing and unlocks BIMI — but rushing it blocks real mail. A staged, reports-driven path from p=none to p=reject, updated for DMARCbis (RFC 9989) and its removal of the pct tag.
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Best SMTP server for high-volume email in 2026
The honest answer is rarely a single product. The real first decision is whether to send it yourself at all — ESP, managed dedicated server, or self-hosted MTA — and the choice turns on volume, horizon, and how much deliverability work you want to own.
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Rent vs buy a GPU server: the honest economics for 2026
The raw break-even math says buy; the real math — utilization, depreciation, power, and obsolescence — usually says rent. A sourced look at when ownership genuinely wins, when it doesn't, and why most teams land on a hybrid.
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KumoMTA vs PowerMTA: an honest comparison for high-volume senders
Both can move millions of messages a day. The real choice is about licensing, support model, existing investment, and volume — and for many senders the honest answer is neither. A sourced, plain comparison, including where Postfix is enough.
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More notes are on the way — deliverability, GPU economics, and network design next.