Email Deliverability Guide: How to Improve Inbox Placement

To improve email deliverability, work four levers in order: authenticate your domain with SPF, DKIM and an aligned DMARC record; keep your list clean so bounces and spam traps stay near zero; control volume, ramping new domains and IPs gradually; and earn engagement by mailing people who want your mail and making it easy to leave. Then monitor spam rate and bounces so you notice problems in days, not months.

Deliverability means reaching the inbox. It isn't the same as delivery, which only means the receiving server accepted the message; accepted mail can still go to spam. This guide covers each lever, what the big three mailbox providers require, how to monitor, and what to do when placement drops. Each section links to a deeper post.

Check your current state first: the free deliverability audit checks SPF, DKIM, DMARC and MX for any domain in one pass, and the blacklist check shows whether your sending IP or domain is listed.

How mailbox providers decide where mail goes

Every incoming message is judged on who sent it, how that sender has behaved, and how recipients have reacted to it. The exact models are private, but the inputs providers publish guidance about fall into a few groups:

• Identity: does the message authenticate, and does the authenticated domain match the From address? Without this, nothing else about your history can be attached to the message reliably.
• Reputation: the track record of your sending domain and IP. Bounces, spam-trap hits, complaints and sudden volume changes all count against it, and it builds slowly.
• Recipient reactions: spam reports, and at large providers other signals of whether people want the mail. Google publishes the spam-rate side of this in Postmaster Tools.
• The message itself: links and the domains they point to, formatting, and whether it looks like what the sender usually sends.

The practical consequence is that reputation is earned per domain over time, and one bad send to a bad list can undo months of good sending. That's why the levers below come in this order: identity first, then the list, then how fast you send, then how people respond.

Lever 1: Authentication

Authentication proves a message really came from your domain, and Gmail, Yahoo and Microsoft now require it at volume. It's necessary but not sufficient: it tells a provider who sent the mail, not whether anyone wants it. SPF, DKIM and DMARC explained covers all three in one place, and DMARC record examples has copy-paste records.

The three records:

• SPF lists the servers allowed to send for your domain. Check it with the SPF checker or build one with the SPF generator. The most common failure is the 10-lookup limit; see fixing "too many DNS lookups" and common SPF mistakes in 696 domains.
• DKIM signs each message with a key published in DNS. Use the DKIM checker, and see setup for Google Workspace and Microsoft 365, plus key length and rotation.
• DMARC tells receivers what to do when SPF and DKIM don't align with the From domain, and sends you reports. Generate one with the DMARC generator, check it with the DMARC checker, then move from p=none to p=reject once reports are clean.

Supporting pieces: a reverse DNS (PTR) record for your sending IP, which Google requires of all senders; ARC for forwarded mail; and BIMI, which shows a logo but only after DMARC enforcement.

A common gap is alignment: SPF and DKIM pass, but for the platform's domain rather than yours, so DMARC fails. Send through a custom sending domain on your ESP.

What Gmail, Yahoo and Microsoft require

All three converge on authentication, low complaints and easy unsubscribing for high-volume senders. The detail differs, and the differences matter.

Requirement	Gmail	Yahoo	Outlook.com
Who counts as bulk	Over 5,000 messages a day to personal Gmail	"Significant volume", no number	Over 5,000 a day per domain
Authentication	SPF and DKIM, DMARC at least p=none, aligned	SPF and DKIM, DMARC at least p=none	SPF and DKIM pass, DMARC at least p=none, aligned
Spam rate	Below 0.3%, aim below 0.1%	Below 0.3%	No published figure
One-click unsubscribe	Required for marketing mail	Required	Not required; a functional unsubscribe link recommended

Gmail figures are from Google's sender guidelines, read on 8 October 2026. For the full side-by-side, see Gmail and Yahoo bulk sender requirements and Microsoft Outlook's sender requirements, which also covers the 550 5.7.515 rejection. Google also asks every sender, bulk or not, to use SPF or DKIM, valid forward and reverse DNS, and TLS.

One-click unsubscribe has a specific technical form; see implementing RFC 8058 one-click unsubscribe.

Lever 2: List quality

Most deliverability problems start with who is on the list, not what the email says. Invalid addresses produce hard bounces, old addresses can become spam traps, and people who never asked for your mail report it. For what counts as a normal rate, see email bounce rate benchmarks and hard vs soft bounces.

• Verify before sending. The free email verifier checks single addresses, and the list health check finds duplicates, typos and dead domains in a CSV. How to clean an email list gives the full process.
• Know the thresholds. Each sending platform acts at a different bounce rate; see bounce rate limits by platform and how to calculate bounce rate.
• Handle bounces correctly. Suppress hard bounces at once and retry soft bounces only within limits; see handling soft bounces and what mailbox providers say when a mailbox doesn't exist.
• Keep spam traps off. They come from typos, old lists and scraped data; see how spam traps get on your list.
• Stop bad addresses at signup with double opt-in, real-time verification and typo correction.
• Re-verify on a schedule. Lists decay; see how often to re-verify, and mailing an old list if one has sat unused.
• Don't buy lists. See should you buy an email list.

Email list management ties these into a six-stage lifecycle.

Lever 3: Volume and infrastructure

Mailbox providers trust consistency: steady volume from a known domain and IP. Sudden spikes from a new or quiet sender look like compromise or spam.

• Warm up new domains and IPs with a small daily volume to your most engaged recipients, increasing gradually. No major provider publishes a schedule; ours is in email warm-up schedule.
• Respect sending limits. See Gmail sending limits, and for cold outreach, how many cold emails per inbox per day.
• Pick the right IP setup. A dedicated IP only helps with enough steady volume to build its own reputation; see shared or dedicated IP.
• Separate streams. Keep cold outreach off your main domain (secondary domains for cold email), and consider separate subdomains for marketing and transactional mail.

Lever 4: Engagement and complaints

Complaints are the strongest negative signal you control. Google's 0.3% limit is measured against mail delivered to the inbox: at 20,000 inboxed messages a day, that's 60 people clicking "Report spam".

What keeps complaints down:

• Send only to people who expect it, and remind them why they're getting it.
• Make leaving easier than reporting. One-click unsubscribe plus a visible link.
• Retire inactive subscribers. A sunset policy removes people who stopped engaging; a re-engagement campaign gives them one last chance first.
• Segment by mailbox provider to spot problems at one provider early; see segmenting by MX record.

Content matters less than people assume. Individual "spam words" carry little weight; links, domain reputation and recipient reactions carry much more. See do spam trigger words still matter. For tab placement rather than spam, see the Gmail Promotions tab.

Monitoring: see problems early

Watch a few numbers weekly, and look at them per mailbox provider.

• Google Postmaster Tools for spam rate and compliance status; see reading Postmaster Tools.
• Bounce rate per send, from your ESP.
• Blocklists, with the blacklist check; Spamhaus lists explained shows which listings matter.
• IP reputation scores such as Sender Score, with caution about what they measure.
• Seed tests for a placement snapshot, read honestly; see inbox placement seed tests.
• Headers of a test message, which show authentication results; use the email header analyzer and how to read email headers.

A falling spam rate alongside falling opens and replies can be a warning rather than good news: if most mail already goes to spam, few people see it in the inbox to report it.

When deliverability drops

Stop or slow sending, find the cause, fix it, then return gradually. Sending more to "push through" makes it worse.

1. Bounces spiked? Pause and verify. See what to do after a high bounce rate.
2. Landing in spam at one provider? Diagnose by provider: Gmail or Outlook.
3. Listed on a blocklist? Fix the cause, then request removal; see how to get off a blacklist.
4. Reputation damaged? Follow a realistic 30-day recovery plan.

Where to start if you only have an hour

If deliverability is slipping and you can only do a few things today, do them in this order:

1. Run the deliverability audit on your sending domain and fix any missing or failing SPF, DKIM or DMARC record.
2. Pull your last three sends' bounce rates. If any is above your platform's threshold, verify the list before the next send.
3. Open Google Postmaster Tools and check the spam-rate trend for the past month.
4. Remove subscribers with no engagement in the past six months or so from your next campaign, pending a re-engagement send.
5. Check the blacklist check result and act on any listing that matters.

Checklists and tools

For a pre-send run-through, the email deliverability checklist has 25 checks. For tooling, email deliverability tools sorts them by job. If you send cold email, the complete cold email guide puts deliverability in the context of the whole campaign.

Of the four levers, list quality is the one you can fix in an afternoon. Start with a sample in the free verifier; SimpleVerifier's monthly plans for full lists are on the pricing page.