Skip to content
InboxLee
Guide · deliverability

Cold email deliverability, the whole picture.

Deliverability is whether your cold email reaches the inbox rather than the spam folder or the void. It is not one setting. It is the sum of authentication, domain and mailbox reputation, sending volume and distribution, list quality, and how mailbox providers have learned to treat your patterns over time.

Two variables do most of the damage when senders get them wrong. The first is mailboxes per domain: stacking four or more onto a single domain concentrates every reputation problem in one place, so one bad mailbox drags the rest down with it. That is why InboxLee enforces a 2-to-3 ceiling at checkout. The second is daily volume: 10 to 20 cold sends per mailbox per day is the safe band, and sustained sending above it is where senders usually get into trouble, especially in the first month after warm-up.

Everything else is reputation management: warm a fresh domain before you send commercially, keep your lists clean so bounces stay low, monitor blacklists daily, and diagnose placement drops fast before they compound across a cohort. This guide groups the playbooks by the job you are doing, whether you are planning capacity, warming up, or pulling a domain out of the spam folder.

Placement and reputation

The metrics that decide whether you land, the mailboxes-per-domain ceiling, and how the big providers score you.

Deliverability · 5 minInbox placement vs delivery rate: stop optimising the wrong numberDelivery rate is a fictional metric. Inbox placement is the only one that matters. Here is the difference and why it costs operators their pipeline.Read →Deliverability · 8 minWhy 2-to-3 mailboxes per domain is the deliverability ceiling nobody tells youWhy InboxLee refuses to provision a fourth mailbox on a single domain, what the domain-level signal actually is, and how to plan a ramp around it.Read →Infrastructure · 5 minHow many mailboxes do you actually need for cold email?A simple formula based on daily send volume, ramp time, and the 50-per-day Gmail ceiling.Read →Deliverability · 7 minHow to recover cold-email sender reputation after a deliverability dropReputation recovery is slow, measurable, and reversible. A 30-day playbook for taking a flagged domain back to Primary inbox.Read →Deliverability · 6 minGmail vs Outlook cold-email placement: what changes when your recipients use different providersWorkspace and M365 use different spam filters, different reputation systems, and different sender requirements. Here is how cold-email placement diverges between them.Read →Deliverability · 6 minWhy your cold email lands in Gmail Promotions (and what actually moves it to Primary)Promotions is not spam. It is also not where most B2B cold email gets answered. Six signals Gmail uses to decide, and which ones you can move.Read →

Warm-up and ramp

How warm-up actually works, how long a new domain needs, and why propagation and domain age matter before you send.

Infrastructure · 6 minMailbox warmup mechanics: what actually happens during the 14 daysWarmup is not magic. Here is what providers see and how they decide your reputation is real.Read →Infrastructure · 6 minHow to warm up a cold-email domain in 2026: the 30-day playbookDomain warmup is different from mailbox warmup. The domain takes 30 days; the mailboxes take 14. Here is what to do during each.Read →Deliverability · 6 minDomain age and cold email: how new is too newAged domains help. Brand-new domains hurt. The transition between the two is sharper than most operators expect.Read →Infrastructure · 5 minDNS propagation timelines: when "live" is actually livePropagation is not instant. Here is how long each step takes and what to expect during the wait.Read →

Diagnose and fix

When mail lands in spam, bounces, or hits a blacklist: read the signals, seed-test placement, and recover.

Guides · 7 minWhy your cold email is going to spam: a diagnostic checklistSix common causes, in order of frequency, with the dig commands to confirm each one.Read →Deliverability · 5 minSoft bounces vs hard bounces: how each one hurts your sender reputationBoth are bad. They hurt in different ways, and the response strategy is different for each.Read →Infrastructure · 6 minSMTP bounce codes explained: 4xx, 5xx, and what each one tells a cold-email operatorEvery cold-email bounce returns a numeric SMTP code. Most operators ignore them. Reading the codes turns a black-box bounce into a specific problem you can fix.Read →Infrastructure · 5 minEmail greylisting in 2026: what it is and how it affects cold-email deliveryGreylisting is a 20-year-old spam-prevention trick that some receivers still use. It introduces a 5-30 minute delay on first delivery and shows up in your logs as a soft bounce.Read →Deliverability · 6 minSeed list testing for cold email: how to actually measure inbox placementSeed lists are the only way to measure inbox placement directly. Here is how to build one that returns real signal.Read →Deliverability · 7 minHow to get a cold-email domain delisted from a blacklistListings on Spamhaus, SURBL, Barracuda and the rest. Each list has its own delisting path. Some clear in hours; some take two weeks.Read →Deliverability · 5 minSpamhaus SBL vs DBL: which list is which and what they each blockSpamhaus runs multiple blacklists. The SBL blocks IPs, the DBL blocks domains, and the consequences for cold email are very different.Read →Deliverability · 8 minThe 2026 cold-email deliverability checklist: 17 things to verify before scalingComprehensive pre-scale checklist for cold-email infrastructure. Every item is binary - pass or fix - with the dig command or test to confirm.Read →
FAQ

Questions, answered