You send an email and it never arrives, or it lands in the recipient's spam folder instead of their inbox. From the recipient's side it looks like you simply did not contact them; from your side, the message shows as sent. The gap between "sent" and "delivered to inbox" is what email deliverability describes — and it is not a mystery. Mail servers make deliverability decisions based on a set of signals that are largely documented and predictable. Understanding them explains every common deliverability failure: why a legitimate newsletter ends up in promotions, why a transactional confirmation goes to spam, why a cold email never arrives at all, and why adding a DNS record can fix a problem that content changes cannot. This guide covers every major signal in plain language.
What Deliverability Actually Means
Deliverability has a precise definition in the email industry: the rate at which messages arrive in the inbox rather than being rejected, quarantined to spam, or silently discarded. A message can fail at multiple points in the delivery chain. The sending server might be rejected outright at connection, meaning the message never reaches the destination server at all. It might be accepted by the destination server and then quarantined to spam. It might reach the inbox but be filtered into a tab or category (Gmail's "Promotions" tab, for example) that reduces open rates significantly. Each of these outcomes has different causes and different fixes. True deliverability is the percentage of sent messages that reach the primary inbox — accepted, not spam-folded, and not filtered to a secondary tab.
Authentication: The Foundation That Everything Else Rests On
The most consistent deliverability fix is proper email authentication, and it is the first thing to check when anything is going wrong. The three authentication standards — SPF, DKIM, and DMARC — tell receiving mail servers that a message is genuinely from the domain it claims, and their absence or misconfiguration is treated as a red flag by every major provider. We explain what each one does technically in reading email headers, but the practical summary for deliverability is: if your DNS records are not properly configured for all three, no amount of content optimisation will fully solve your inbox placement.
SPF (Sender Policy Framework) requires you to publish a DNS TXT record listing the IP addresses authorised to send mail for your domain. DKIM (DomainKeys Identified Mail) requires your sending server to sign each outgoing message with a private key, and publish the corresponding public key in DNS so receivers can verify the signature. DMARC (Domain-based Message Authentication, Reporting, and Conformance) requires you to publish a policy that tells receivers what to do when SPF and DKIM fail, and provides a reporting mechanism so you can monitor authentication results. A complete, correctly configured setup with all three reduces the probability of spam-folding significantly compared to sending with no authentication, and it is required for Gmail and Yahoo's bulk sender guidelines, which mandate SPF, DKIM, and DMARC for any sender sending more than 5,000 messages per day to Gmail addresses.
IP and Domain Reputation
Every IP address and every domain has a reputation score that mail filters maintain and update based on the sending behaviour observed from that address or domain over time. Sending high volumes of email that recipients mark as spam lowers reputation. Hitting spam traps — email addresses that are not real user accounts but are seeded into lists to catch senders who scraped or purchased rather than organically acquired their list — is a severe reputation penalty. Sending to many invalid addresses that bounce with 550 hard-bounce errors is another signal of a low-quality list and degrades reputation over time.
IP reputation and domain reputation are separate signals. Shared IP addresses — used by most small senders on email service providers — mean that other senders on the same IP affect your reputation. This is why reputable email service providers (ESPs) invest heavily in list hygiene and sending-practice enforcement: a single bad actor on a shared IP can damage deliverability for legitimate senders. Dedicated IPs, which larger senders often use, isolate your reputation from other senders but require a "warmup" period during which you gradually increase sending volume so that receiving servers can establish a reputation for the IP based on engagement data.
Domain reputation works similarly. A domain that has been used for spam campaigns carries that history in reputation databases, and switching to a fresh domain does not instantly give you good standing — you have to build it through consistent, low-complaint sending. For new domains, starting with small volumes and gradually increasing them (warmup) is standard practice. For established domains, protecting reputation means never purchasing or scraping email lists, keeping hard-bounce rates below 2%, and maintaining spam complaint rates below 0.1% (Google's recommended threshold).
List Quality and Engagement Signals
Modern spam filters at Gmail, Microsoft, and Yahoo incorporate recipient engagement as a major deliverability signal. When recipients open your messages, click links, and move them out of spam into their inbox, those signals tell the filter that the messages are wanted. When recipients delete without opening, scroll past without interacting, or mark as spam, those signals indicate unwanted mail. Over time the filter learns from this behavior and adjusts future placement for that sender accordingly.
This means that list hygiene — the practice of regularly cleaning your mailing list of unengaged subscribers, invalid addresses, and likely spam traps — is not just tidiness but a direct deliverability tool. Sending to a smaller, more engaged list typically produces better inbox placement than sending to a large, stale list with low engagement, even if the content is identical. Subscribers who have not opened your emails in six months or more are actively harming your reputation if they continue to receive (and not engage with) your messages. Standard practice is to run re-engagement campaigns for inactive subscribers and remove those who do not re-engage.
Content Signals: What Filters Actually Look For
Content filtering has evolved significantly from the early days of spam, when simple keyword matching (words like "free", "click here", "make money") was the primary tool. Modern content filters use Bayesian classifiers, neural networks, and multi-signal analysis rather than simple keyword lists. Nevertheless, certain content patterns remain strong signals of spam: excessive use of urgent language, messages composed of a single image with no text, very high link density, URLs that mismatch anchor text, and HTML that contains hidden text or unusual encoding are all patterns associated with bulk spam campaigns and scored accordingly.
Practically, the most important content principles for deliverability are: include meaningful plain-text alternative content alongside any HTML (some filters score HTML-only messages lower), keep the text-to-image ratio reasonable rather than embedding all content in images, make sure all links in the message go to the domain you are sending from or closely related legitimate domains, and avoid patterns common in spam like excessive punctuation in subject lines, ALL CAPS subjects, and misleading preview text. Having a clear, honest subject line and a message body that delivers what the subject promises also improves engagement rates, which in turn improves reputation.
Sending Infrastructure and Technical Configuration
The technical setup of the sending server matters beyond authentication. Reverse DNS (rDNS) requires that the IP address your mail server sends from has a PTR record that resolves to a hostname, and that hostname resolves back to the IP — this bidirectional check (forward-confirmed reverse DNS, or FCrDNS) is required or strongly preferred by many mail servers. Sending from an IP address without a properly configured PTR record is a strong spam signal. Your mail server should also identify itself correctly in the SMTP EHLO/HELO greeting — using a fully qualified domain name that matches or relates to your sending domain, rather than a bare IP or localhost.
The reputation of the email service provider (ESP) you use also matters. ESPs like SendGrid, Amazon SES, Mailchimp, Postmark, and others maintain relationships with major mail providers and actively monitor and enforce sending practices on their networks. Sending through a reputable ESP gives you access to their reputation infrastructure and their pre-existing relationships with Gmail, Microsoft, and Yahoo. Sending from a self-hosted server requires you to build that reputation from scratch and maintain it yourself.
Why Disposable Email Domains Are Sometimes Blocked
From a deliverability perspective, some senders block incoming sign-up registrations from known disposable email domains, as we discuss in why websites block disposable email. From a deliverability perspective, this is a defensive measure against a specific kind of list inflation: if people sign up with disposable addresses that expire before the first sending, those addresses immediately hard-bounce, and a list with many hard bounces has poor deliverability metrics. Blocking disposable domains at registration prevents these addresses from entering the list in the first place. For users, this means that disposable inboxes work for receiving one-off confirmation emails but cannot always be used as the long-term address on an active mailing list subscription — the list owner's interest in list hygiene and your interest in inbox privacy are in tension, and the block is the list owner's solution to that tension.
How to Diagnose a Deliverability Problem
When messages are not reaching inboxes, the diagnostic order is: first, check authentication (use MXToolbox's SPF, DKIM, and DMARC check tools to verify your DNS records are correctly published and valid); second, check your sending IP and domain against major blocklists (MXToolbox Blacklist Lookup covers the most important ones); third, send a test message through a deliverability testing service like Mail-tester.com or GlockApps, which scores your message and reports on authentication, content, and reputation signals; fourth, if you use an ESP, check their reputation and deliverability reports for your sending domain. Google Postmaster Tools (a free service for senders to gmail.com recipients) provides domain reputation scores, spam complaint rates, and authentication reports that are invaluable for diagnosing Gmail-specific deliverability problems.
The Short Version
Email deliverability depends on three overlapping sets of signals: authentication (SPF, DKIM, DMARC correctly configured in DNS), reputation (IP and domain reputation built through clean lists, low bounce rates, and low complaint rates over time), and content and engagement (message content that does not pattern-match spam, and recipients who open and interact rather than delete or mark as spam). Fixing a deliverability problem almost always starts with authentication, because a message that fails SPF, DKIM, or DMARC is treated as suspicious regardless of content or reputation. Maintaining reputation requires consistent list hygiene and not sending to people who never opted in. And engagement signals mean that a highly targeted, relevant message to a small engaged list will often outperform a large blast to a stale one. For senders, Google Postmaster Tools provides ongoing visibility into how Gmail perceives your sending domain, which is the most actionable single monitoring tool available.
Frequently Asked Questions
Why is my email going to spam even though I have SPF and DKIM?
SPF and DKIM passing removes one class of spam signals but not all of them. Spam filters also weigh IP and domain reputation, recipient engagement history, content patterns, and whether your sending practices have produced bounce or complaint rates above thresholds. If you have correct authentication but poor inbox placement, check: whether your IP is on any major blocklists (MXToolbox Blacklist Lookup), whether your domain reputation is degraded from past high-complaint sending (Google Postmaster Tools shows domain reputation for Gmail recipients), and whether your content contains patterns associated with spam. Also verify DMARC is configured, not just SPF and DKIM — DMARC is the policy layer that makes authentication enforceable.
What is a spam trap and how does hitting one affect me?
A spam trap is an email address seeded into the web or into old lists specifically to catch senders who scrape or purchase addresses rather than collecting them through genuine opt-in. There are two types: pristine traps (addresses that never belonged to a real person, only ever placed online as bait) and recycled traps (addresses that belonged to real users, were abandoned, and have been repurposed as traps after a period of inactivity). Hitting pristine traps is a severe signal — it means your list has addresses that could only have come from scraping — and can result in IP or domain blacklisting. Hitting recycled traps indicates you are sending to addresses that have not engaged for an extended period and should have been removed.
Does email deliverability affect temp mail inboxes?
Yes, from the receiving side. A message sent to a temp mail address goes through the same deliverability checks any message does — the sending domain's authentication is verified, the sending IP's reputation is checked, and the message content is evaluated. A well-authenticated, legitimate sender (like a major website sending you a confirmation email) will reach a temp mail inbox reliably. A poorly authenticated sender or one with a bad IP reputation may be filtered or rejected by the temp mail provider's own spam checks, just as it would be at Gmail. From the sending side, senders who check whether recipient addresses are on disposable-email blocklists may reject a temp mail address at registration before a message is ever sent.
What is the recommended spam complaint rate for good deliverability?
Google's published guidance for Gmail deliverability is to keep spam complaint rates below 0.10%, with any rate above 0.30% leading to significant inbox placement problems. Yahoo has published similar thresholds. These rates are measured by your email service provider through feedback loops with Gmail and Yahoo that report when recipients mark your messages as spam. Rates above these thresholds indicate that recipients find your messages unwanted, which triggers lower inbox placement, which typically further reduces engagement, in a negative spiral. The fix is always to remove unengaged subscribers, improve targeting so you send only to people who want the content, and ensure that every subscriber on your list genuinely opted in.
Achyuth Kumar
Founder & editor, TempMailKit
Achyuth builds privacy tools and writes TempMailKit’s guides on email security, spam, and online privacy. Every article is checked against primary sources and our editorial policy before it is published. Questions or a correction? Get in touch.