What deliverability actually means
Deliverability is the question of whether your email lands in the inbox, gets filtered to spam, or is rejected before it arrives. It is distinct from delivery. A message can be "delivered" to a mailbox provider and still be quietly routed to spam. For a startup, the gap between sent and seen is where revenue leaks. Three levers move it: authentication, reputation, and hygiene. Each one is a mechanism you can verify, not a vague best practice.
Authentication: proving you are you
Mailbox providers will not trust an unverified sender. Three DNS records establish your identity. SPF lists which servers may send for your domain. DKIM cryptographically signs each message so it cannot be tampered with in transit. DMARC tells receivers what to do when SPF or DKIM fails, and asks them to report abuse back to you. Set all three on your sending domain before you send a single campaign. One caveat that trips up most startups: authentication is necessary, not sufficient. Passing all three proves identity and gets you in the door, but placement still depends on reputation and engagement. You can be perfectly authenticated and still land in spam if recipients ignore or report you.
Sender reputation and domain warmup
Reputation is a provider's running judgment of how recipients react to your mail. A brand-new domain has none, so you warm it up: start at low volume and increase gradually. Postmark's guidance is to begin with roughly 50 to 100 messages per major provider in the first days, then grow incrementally, with most domains reaching dependable full-volume deliverability in about three to six weeks. Send your most engaged recipients first to build a positive signal faster. There is no shortcut. Warmup is gradual by design.
The Gmail and Yahoo bulk-sender rules
In February 2024, Gmail and Yahoo set shared requirements for bulk senders, defined by Google as those sending 5,000 or more messages a day to Gmail. The rules are concrete: authenticate with SPF, DKIM, and DMARC; support one-click list-unsubscribe via the right headers; and keep spam complaints low. Google states you should keep the spam rate in Postmaster Tools below 0.10%, and never let it reach 0.30%. Cross 0.30% and you risk filtering and rejection. As of 2025, enforcement has tightened, so build to the rule even below 5,000 a day.
List hygiene, suppression, and engagement
The cheapest deliverability win is not sending to people who do not want your mail. Remove hard bounces immediately, suppress addresses that complain, and prune contacts who have not opened anything in months. Honor unsubscribes the moment they arrive. Mailbox providers watch what recipients do, opens, replies, archives, and above all spam reports, so sustained engagement keeps you in the inbox while a wave of ignored mail pulls you out. Good retention and win-back flows do double duty here, lifting revenue while protecting reputation. This is also where deliverability meets compliance: when you self-host your platform, consent records and suppression lists stay in your own database, so you can prove who agreed to what.
Where deliverability fits in your stack
Deliverability is not a one-time setup; it is an operating discipline that a customer-engagement platform should support directly, with authentication checks, suppression handling, and per-domain reputation monitoring built in. A separation worth keeping clear: transactional and marketing mail have different reputations and rules, so route them on separate streams. Whether you self-host or use a hosted tool such as Brevo or Klaviyo, the same three levers, authentication, reputation, and hygiene, decide whether your mail is seen.