Email deliverability: how spam filters work

Email providers now use artificial intelligence to check every email: who sent it, how it is written, and what attachments, links and images it holds. Anything suspicious is filtered out before it reaches the inbox. Here is how to make sure your emails pass.

1. Your technical records: DMARC, SPF and DKIM

These three records shape how spam filters treat your emails.

SPF lists the servers allowed to send for your domain. If SPF fails, your email is at high risk of rejection or spam. Gmail, Outlook and Yahoo use SPF in their spam score.

DKIM is a digital signature proving your email was not changed after sending. Without it, providers trust your email less. A valid signature improves your score and builds trust in your domain.

DMARC tells receiving servers what to do if SPF or DKIM fails, and protects against spoofing. Without it, filters are more suspicious. With it, you pass checks far more easily.

How filters combine them:

• SPF or DKIM fails and DMARC is strict: the email may be rejected straight away.
• SPF and DKIM pass but DMARC is missing: content is checked more strictly, with a higher chance of Promotions or spam.

In short: SPF is who may send, DKIM is proof the email was not changed, DMARC is what to do on failure plus spoofing protection. Together, they are your passport through the filters, before your content is even read. See How to authenticate your domain name for email sending.

2. Your content

Filters scan your email word by word, image by image and link by link.

2.1 Your sender address

Use your own authenticated domain. Avoid free addresses such as @hotmail.com for sending to your list. They are poorly seen, and DMARC policies may block them. Use your own domain with SPF, DKIM and DMARC set up. Avoid odd or brand new subdomains, such as promo2025-yoursite.com, which look untrustworthy and get blacklisted quickly.

Avoid no-reply addresses. They block replies and lower engagement, which harms your reputation. Choose a friendly address which invites conversation, such as hello@yoursleepbusiness.com or yourname@yoursleepbusiness.com.

Match your sender name and address. "Sarah from Sweet Dreams Sleep" with sarah@sweetdreamssleep.com builds trust. A name with an address on a different domain looks suspicious.

Separate your email types. Use different addresses for different purposes, such as invoices@, newsletter@ and support@ on your domain. A problem with one type then does not hurt the others.

Respect domain age. Filters are cautious with new domains and addresses. Never send large volumes right after creating your domain. Warm it up gradually.

2.2 Your subject line

Avoid spam trigger words: free, gift, urgent, 100% guaranteed, winner, special offer, promotion, cash, income, crypto. Disguised versions such as FR€€ or Fr33 are easy for filters to spot. Pushy phrases such as "Don't miss out", "Last chance" or "Act now" hurt too.

Go easy on punctuation and symbols. Avoid !!!, ??, ?!?, lots of $, % or emojis, and all caps. One or two capitalized words are fine, a whole subject in capitals looks aggressive.

No clickbait. Promises which sound too good to be true, such as "Your baby will sleep 12 hours tonight", raise your spam score. Misleading subjects frustrate parents, who then click Mark as spam. Never use a subject like "Your invoice is ready" for a promotional email. SleepPRO™ has a filter which blocks misleading subject lines, to protect your reputation.

Be clear, honest and specific, and use A/B tests to find what works. See How to set up an A/B test for your newsletters.

2.3 Your sender name

Keep it consistent and recognizable. Do not switch between names like "Support", "Newsletter" and "Promo team".

No spam words, capitals or symbols in your sender name, such as "FLASH SALE !!!". Google is highly sensitive to special characters here.

Match it to your address, for example "Julie from Little Dreamers" with julie@littledreamers.com.

2.4 Your email body

Size: most HTML emails are 30 to 60 KB. Over 100 KB is heavy and risks rejection. Keep your emails under 100 KB in total.

Images:

• Avoid emails made only of images, or with little text. Aim for at least 60% text and at most 40% images.
• Host images somewhere reputable. Free or unknown image hosts are often blacklisted.
• Keep emails 600 to 650 pixels wide, and compress images before sending.
• Use JPEG for photos, PNG for logos and graphics with transparent backgrounds, and animated GIFs sparingly, compressed.
• Never hide text inside images. Filters notice.

Links:

• Watch the balance of links to text. Three short lines with four links looks suspicious. One to three links is plenty for most emails: your main call to action, your website and the unsubscribe link.
• No link shorteners such as bit.ly or tinyurl. Use links on your own domain.
• Repeating a link, for example a button at the top and bottom, is fine. More than three times in a short email looks suspicious.
• Too many links to different outside domains raises suspicion, especially if any have a poor reputation. Links to images hosted elsewhere count as links too.

2.5 Attachments

Attachments can raise your spam score or get your email rejected, especially if they contain anything suspicious. Links inside them are checked too, even at the moment someone clicks.

• Keep each email under 5 MB in total. Many servers block emails over 10 MB.
• Safe formats: .pdf, .jpg, .png and .txt.
• Risky formats, often blocked: .exe, .bat, .js, .scr, and .zip files with programs or macros inside.
• .docx and .xlsx usually pass, but files with macros may be rejected.
• Scan files with an online virus checker before sending.

Following these steps gives your emails the best chance of landing safely in parents' inboxes.