1. Missing or Incorrect SPF, DKIM, DMARC

1. Missing or Incorrect SPF, DKIM, DMARC

These are the biggest causes.

SPF

SPF tells receiving mail servers which servers are allowed to send email for your domain.

Example DNS TXT record:

v=spf1 include:sendgrid.net ~all

(or Mailgun, SES, Postmark, etc. depending on your provider)

Check SPF

Use:

• MXToolbox SPF Check
  

• Google Admin Toolbox CheckMX
  

DKIM

DKIM signs outgoing emails cryptographically.

Without DKIM:

• 
  Gmail heavily penalizes
  

• 
  Outlook often sends to junk
  

• 
  Microsoft 365 distrusts mail
  

Your provider usually gives CNAME/TXT records to add.

Example:

selector._domainkey.marketing.inc

DMARC

DMARC tells recipients what to do if SPF/DKIM fail.

Example starter policy:

v=DMARC1; p=none; rua=mailto:dmarc@marketing.inc

Later move to:

p=quarantine

then eventually:

p=reject

Check:

• MXToolbox DMARC Check
  

2. Your Domain Is New

If marketing.inc is newly purchased:

• 
  Gmail distrusts it initially
  

• 
  Microsoft especially distrusts new domains
  

• 
  Low sending reputation
  

Fix

Warm up the domain gradually.

Do NOT:

• 
  send bulk emails immediately
  

• 
  send hundreds at once
  

• 
  send identical messages
  

Instead:

• 
  start with 10–20/day
  

• 
  slowly increase
  

• 
  ensure recipients open/reply
  

3. Using Shared SMTP Providers With Poor Reputation

If using:

• 
  cheap SMTP relays
  

• 
  shared cPanel mail
  

• 
  unknown VPS mail servers
  

their IP reputation may already be bad.

Better Providers

Use reputable transactional providers:

• Amazon SES
  

• Postmark
  

• SendGrid
  

• Mailgun
  

• Resend
  

For app/system emails, Postmark and SES are especially good.

4. Your Server IP Has Bad Reputation

If you self-host SMTP:

• 
  VPS IP may be blacklisted
  

• 
  reverse DNS may be missing
  

• 
  HELO hostname mismatch
  

Check:

• MXToolbox Blacklist Check
  

5. No Reverse DNS (PTR Record)

Huge issue for self-hosted mail.

Your mail server IP must resolve back to a hostname.

Example:

IP -> mail.marketing.inc

and:

mail.marketing.inc -> same IP

Without this:

• 
  Microsoft junk filters trigger instantly
  

• 
  Gmail distrust increases
  

6. Content Looks Spammy

Common triggers:

• 
  ALL CAPS
  

• 
  too many links
  

• 
  URL shorteners
  

• 
  “Click here urgently”
  

• 
  too many images
  

• 
  no plain-text version
  

• 
  attachment-heavy emails
  

7. Sending HTML-Only Emails

Always send:

• 
  HTML version
  

• 
  plain-text alternative
  

Most email libraries support multipart emails.

Example in NodeMailer:

await transporter.sendMail({
  from: 'support@marketing.inc',
  to: user.email,
  subject: 'Welcome',
  text: 'Welcome to our platform',
  html: '<h1>Welcome</h1>'
});

8. Missing List-Unsubscribe Header

For bulk/system emails, Gmail likes this.

Example:

List-Unsubscribe: <mailto:unsubscribe@marketing.inc>

9. Low Engagement

If users:

• 
  never open
  

• 
  delete immediately
  

• 
  mark spam
  

your reputation worsens.

Especially bad if:

• 
  you imported old email lists
  

• 
  users didn’t opt in
  

10. Your .inc TLD Could Be Treated More Strictly

Some newer TLDs (.xyz, .online, .inc, etc.) get more scrutiny than:

• .com
  

• .org
  

• .net
  

Not necessarily bad, but authentication becomes even more important.

What I Recommend for Your Case

Since you’re building production apps and sending transactional emails:

Best Setup

Use:

• Amazon SES
  
  or
  

• Postmark
  

Configure:

• 
  SPF
  

• 
  DKIM
  

• 
  DMARC
  

• 
  custom tracking domain
  

Verify:

• 
  reverse DNS
  

• 
  domain reputation
  

Test Deliverability

Use:

• Mail Tester
  

This gives:

• 
  spam score
  

• 
  SPF status
  

• 
  DKIM status
  

• 
  blacklist checks
  

• 
  HTML issues
  

Fastest Way To Diagnose

Send a test email to:

• 
  Gmail
  

• 
  Outlook
  

• 
  Yahoo
  

Then inspect:

• 
  “Show original” in Gmail
  

• 
  Authentication results
  

You want:

SPF: PASS
DKIM: PASS
DMARC: PASS

If even one fails, that’s usually the issue.

For Your Node.js Apps

If you tell me:

• 
  which email provider you use
  

• 
  whether SMTP is self-hosted or third-party
  

• 
  your DNS provider (Cloudflare, GoDaddy, Route53, etc.)
  

I can tell you exactly:

• 
  which DNS records to add
  

• 
  how to verify them
  

• 
  how to improve inbox delivery
  

• 
  how to set up production-grade transactional email properly.