How to Avoid Spam Filters: What They Check

A spam filter asks three questions about every message: is the sender who they claim to be, does this sender have a good track record, and does this message look like mail people want. This guide goes through each check and what you can do to pass it. It replaces our two older posts on the spam folder and on keeping mail out of spam; their checklist, provider requirements and FAQs are merged here.

Pre-Send Checklist

Run through it before every campaign:

• SPF, DKIM and DMARC records published and passing for the sending domain

• The From: domain aligned with the SPF or DKIM domain

• New domains and mailboxes warmed up, volume ramped gradually

• List verified; hard bounces and long-inactive contacts removed

• Spam complaint rate below 0.1% in Google Postmaster Tools

• Subject line specific and honest: no fake "Re:", no capitals, no "!!!"

• Message as text (one large image reads as spam); ALT text on every image

• Links point to full URLs on your own domain, no public shorteners

• An easy way to opt out

The sections below explain each item. To see how filters score a specific message, run a spam test.

Check 1: Is the Sender Who They Claim to Be?

Providers such as Gmail and Outlook receive a constant stream of mail that spoofs real domains. Mail that cannot prove where it came from is treated with suspicion. The proof is three DNS records:

• SPF lists the servers allowed to send for your domain (RFC 7208). Include every service that sends for you.

• DKIM signs each message, so the receiver can check it came from your domain unaltered (RFC 6376).

• DMARC ties both to the From: domain and tells receivers what to do with mail that fails: deliver it (p=none), quarantine it or reject it (RFC 9989, which replaced RFC 7489 in May 2026).

Set them up in this order: DMARC relies on SPF and DKIM results. Setup and testing steps: how to test SPF, DKIM and DMARC.

What Gmail, Yahoo and Outlook Require

The three large mailbox providers publish hard rules. The thresholds below come from their own documentation.

Google counts the 5,000 messages across the whole primary domain, subdomains included. Its FAQ recommends keeping the spam rate below 0.1% and never letting it reach 0.3%, and says that since November 2025 Gmail has been ramping up enforcement on non-compliant traffic, including temporary and permanent rejections (Google FAQ). Meeting these rules keeps you from being rejected; it does not put you in the inbox on its own.

Why Custom Tracking Domains Are a Must

If you track opens or clicks, your links are rewritten to go through a tracking domain. Many tools use a shared default domain for this. If another customer on that domain sends spam, the domain can land on a blocklist and your links inherit the problem. A custom tracking domain (a CNAME on your own domain) keeps your link reputation tied to your own sending only.

The same logic applies to public URL shorteners such as bit.ly: they hide the destination, phishers use them, and filters distrust them. Link to full URLs on your own domain.

Check 2: Does the Sender Have a Good Track Record?

Gmail and Outlook keep a reputation for your sending domain and IPs. It is built from what recipients do with your mail: replies, opens, deletions, spam reports. High bounce rates, spam complaints and low engagement lower it over time.

Warm Up New Domains

A new domain has no history, and a sudden burst of mail from it looks like spam. Warm it up:

• Start small. Send a small daily volume to contacts who will open and reply.

• Ramp up slowly. Increase volume step by step over several weeks, and stop increasing when bounces or complaints rise.

• Aim for replies. Replies and real conversations are the positive signals you want.

Warmup tools automate part of this; see our review of the best email warmup tools.

Keep Your List Clean

Dead addresses produce hard bounces, and people who never engage lower your engagement rate. To a provider, both look like a sender using a scraped list.

• Verify addresses before you send: choosing an email verification program.

• Remove hard bounces after the first failure.

• Define inactive for your program (for example, no opens or clicks in 90–180 days), send that segment one re-engagement message, and remove those who do not respond.

• Use double opt-in for newsletters and marketing lists: the address joins only after the confirmation link is clicked. It catches typos and fake addresses and records consent.

• Never buy a list. Bought lists contain dead addresses, spam traps and people who never heard of you.

Drive Down Complaints

A spam report is the strongest negative signal. Google asks senders to keep the spam rate below 0.1% (1 complaint per 1,000 messages) and never reach 0.3% (Google).

1. Make opting out easy. Gmail and Yahoo require one-click unsubscribe (the List-Unsubscribe-Post header from RFC 8058) plus a visible link for marketing mail from bulk senders. For one-to-one outreach, a line such as "Reply 'no' and I won't follow up" does the same job.

2. Meet expectations. Send what people signed up for, or what your first cold email promised.

3. Ask for a reply. End with a question. Replies show a real conversation.

4. Read feedback loops. Yahoo's complaint feedback loop reports each complaint about DKIM-signed mail, so you can suppress that recipient everywhere. Google's Feedback Loop shows aggregate spam rates per Feedback-ID in Postmaster Tools, which tells you which campaign draws complaints.

Send Consistently

Filters compare today's traffic with your history. A dormant domain that suddenly sends a large campaign, or mail that jumps between IPs and formats, looks suspicious. Keep volume, sending infrastructure and templates predictable.

Check 3: Does the Message Look Like Wanted Mail?

Content filters read your subject line, body, HTML and links. There is no loophole: the content that passes is the content that looks like normal correspondence.

Avoid Spam Trigger Words

Some phrases are strongly associated with spam: "Act now!", "Limited time offer", "100% free", "Winner". Filters allow them. A message full of them still looks like spam to filters and to readers. State what you offer in plain words.

Treat this as a starting point. The goal is plain language.

Balance Your Text and Images

An email that is one large image with little text looks like an attempt to hide content from filters. Put the message in text and use images only to support it. Add descriptive ALT text to every image: screen readers and clients that block images show the ALT text instead.

Write Honest Subject Lines

The subject line has to match the email. A fake "Re:" or "Fwd:" to imitate a previous conversation misleads the reader and is a known spam tactic.

• Go easy on punctuation. No "!!!" or "???".

• Do not write in capitals. Use sentence case or Title Case.

• Be specific. "A few thoughts on your Q3 content plan" instead of "Checking in".

• Do not obfuscate. "F.R.E.E G.I.F.T" is a pattern filters look for.

Keep Your HTML Clean and Simple

• Do not paste from Microsoft Word. It adds hidden formatting that breaks layouts.

• Keep emails light. Less code, fewer images, fewer links. For cold outreach, plain text works well.

Filters Look at Patterns as Well as Words

Gmail combines rule-based filters with machine-learning models that weigh many signals per message, and Google has described using them to catch image-based spam and mail from newly created domains (Google). Keyword tricks do not get past that. Consistent, legitimate behavior does.

How to Look Legitimate to Filters

• Keep your voice and format consistent. A sudden change in volume, format or tone from the same domain stands out.

• Vary messages for real reasons. Personalize each email to the recipient. Identical copies sent to thousands of people read as bulk mail.

• Aim for replies. A reply is evidence of a real conversation.

Use AI Writing Tools Carefully

AI tools help with drafts, but unedited AI output tends to read the same across senders. Generic mail gets less engagement, and low engagement hurts reputation. Edit every draft: add specifics about the recipient, cut filler, write in your own voice.

Monitor Deliverability and Fix Issues Fast

Sending is not the end of the job. Watch the numbers per campaign and per provider, and act on changes early.

Metrics to Track

Use Postmaster Tools and SNDS

Google Postmaster Tools shows your spam rate, authentication results, delivery errors and whether you meet Gmail's sender requirements. Postmaster Tools v2 no longer includes the old Domain and IP Reputation dashboards (Google), so rely on the compliance, spam rate and error data. More tools: email domain reputation checkers.

Microsoft offers SNDS (Smart Network Data Services) for the IPs you send from. Read your DMARC reports too: they show which servers send as your domain and which fail authentication.

Split Results by Provider

Each provider runs its own filters and keeps its own view of your reputation, based on how its own users react. A domain can be in good standing with Microsoft and poor standing with Google, or the reverse. Compare bounces, complaints and replies for Gmail recipients against Outlook recipients before you change anything.

What to Do If You Get Blacklisted

1. Find the list. Check which blocklist names your domain or IP, for example with MXToolbox.

2. Find the cause. A complaint spike, a bought or stale list, spam traps, a compromised account. Fix it before you ask for removal.

3. Follow the delisting process. Each blocklist publishes its own; follow it exactly and explain what you changed.

4. Be patient. Removal can take hours or days. Afterwards, send conservatively.

Common Questions About Spam Filters

I Set Up SPF and DKIM. Why Are My Emails Still Going to Spam?

Authentication proves identity; it does not vouch for behavior. You can still be filtered for:

• A weak sender reputation: past bounces and complaints follow the domain.

• Low engagement: people ignore or delete your mail.

• Content: misleading subject lines, image-only emails, shortened links.

How Long Does Warming Up a New Domain Take?

Several weeks. The exact time depends on how fast you raise volume and how recipients respond. When bounces, complaints or replies move the wrong way, stop raising volume until they recover. Rushing it is a common way to damage a new domain.

Is Buying an Email List Ever a Good Idea?

No. Bought lists contain dead addresses, spam traps and people who never agreed to hear from you. Expect bounces and spam complaints from the first send. Build your own list or targeted prospect lists instead.

Do Emojis in Subject Lines Make Me Look Like a Spammer?

Not by themselves. One relevant emoji is fine. A subject stuffed with emojis and capitals looks like spam to people and to filters.

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