Deliverability field notes.
The why-cold-emails-fail-in-2026 corpus. Content fingerprinting, sender reputation, the spam classifiers behind Gmail and Outlook, and the engineering tradeoffs that decide whether your real campaign hits the inbox.
Soft Bounce vs Hard Bounce: What Actually Differs
Soft bounce vs hard bounce — SMTP 4xx vs 5xx, why one is temporary and one is permanent, retry policy, and how each affects sender reputation.
Cold Email vs Spam: The Real Difference (Legal AND Technical)
Cold email vs spam — the legal difference under CAN-SPAM, GDPR, and CASL, and the technical difference mailbox providers enforce regardless of law.
Domain Reputation: How Mailbox Providers Score Your Domain
Domain reputation — what mailbox providers measure, where to check your score (Postmaster, SNDS, Sender Score), and how to repair a damaged sending domain.
Why Your Cold Emails Go to Spam (Even With a Warm Sender)
Sender reputation is only half the equation. Gmail and Outlook fingerprint your message body independently — here is why warm senders still hit spam, and how to fix it.
Sender Score Explained: What It Is, How to Improve It
Sender Score is a 0-100 IP reputation number from Validity. Here is what it actually measures, when it matters, when it does not, and how to improve it.
Cold Email vs Marketing Email: The Real Differences
Cold email vs marketing email — different intent, different infrastructure, different rules, and why mixing them on one domain kills deliverability.