05 The physics of the market · 11 pages · 14k words
Deliverability
Every cold-email platform lives under rules it does not write: how Gmail and Outlook filter, what Google, Yahoo and Microsoft require, and what practitioners have learned the hard way about volume, warmup and tracking.
How filtering works
Reputation, authentication, and the machinery that decides inbox or spam.
How filtering works
Gmail and Microsoft decide inbox vs spam with ML classifiers fed by domain reputation, recipient behaviour and content; published bulk rules are a floor, and most B2B cold email is sent to recipients those rules do not even cover.
Reputation and blocklists
Cold-email reputation lives on domains, not IPs, and the first-party tools that used to show it (Postmaster v1 reputation, SNDS detail) were retired or degraded in 2025–26, leaving senders to infer health from bounces and replies.
Authentication: SPF, DKIM, DMARC, BIMI, ARC and one-click unsubscribe
SPF, DKIM and an aligned DMARC record are mandatory plumbing every platform already automates; BIMI is irrelevant to cold domains, and RFC 8058 one-click unsubscribe is legally ambiguous for cold email but cheap insurance.
The rules
What the mailbox providers require, and how they enforce it.
Google and Yahoo sender rules
Since February 2024 Gmail and Yahoo require authentication, a sub-0.3% complaint rate and one-click unsubscribe from bulk senders; Gmail escalated to hard rejections in November 2025, but the rules only cover personal mailboxes and most cold domains never hit the 5,000/day line.
Microsoft rules: Outlook.com and Microsoft 365 limits
Outlook.com has required SPF, DKIM and DMARC from senders above 5,000/day since May 2025; on the sending side Microsoft cancelled its 2,000-per-mailbox external limit in January 2026, leaving the 10,000-recipient mailbox cap and a licence-scaled tenant limit.
Provider crackdowns
A dated record of Google and Microsoft enforcement that hit cold-email infrastructure — from the 2024 bulk-sender rules to the autumn 2025 Google suspension wave, Microsoft's May 2025 Outlook rules and the 2026 cancellation of its per-mailbox cap.
Practice
Infrastructure choices, volume arithmetic, warmup, content, and honest benchmarks.
Google vs Microsoft vs SMTP
Google Workspace inboxes cost more per email and get suspended more, Microsoft tenants are cheaper per domain but throttled, private SMTP is cheapest and least trusted; most agencies blend all three.
Volume math: what 10k and 100k emails a day cost
At 2026 norms of 10–30 sends per mailbox per day, 100,000 cold emails a day needs roughly 6,500 Google mailboxes on ~2,200 domains (~$25k/month in infrastructure), and burns through 500k+ new contacts a month.
Does warmup work?
The practitioner consensus says ramp new inboxes for 2–4 weeks; the sceptics say automated warmup pools are fake engagement that filters learn to discount or punish. The evidence on both sides is mostly vendor-made.
Content and tracking
Open rates are now mostly noise (Apple MPP, security scanners, Gmail image caching); link tracking adds a shared-domain liability; plain, short, link-light text with real variation is the 2026 default, and content tricks are what Gmail's classifiers are built to catch.
Reply benchmarks
Published 2025–26 cold-email reply rates run from 0.45% (Belkins) to 3.7% (Saleshandy) because vendors use different denominators and reply definitions; most 'benchmarks' recycle Instantly's 3.43%, and positive-reply and meeting rates are barely measured.