How to Prevent Rails Application Emails from Being Flagged as Spam: A Technical Guide for SMTP/Sendmail Configuration


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When your Rails application emails keep landing in recipients' spam folders, it's typically due to email authentication failures or content triggers. Let's examine both technical and content-related solutions.

First, implement these three critical DNS records for your domain:

# Example SPF record (TXT)
v=spf1 include:_spf.google.com ~all

# Example DKIM record (TXT)
v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...

# Example DMARC record (TXT)
v=DMARC1; p=none; rua=mailto:dmarc-reports@yourdomain.com

For Rails applications using Sendmail, ensure proper configuration in config/environments/production.rb:

config.action_mailer.delivery_method = :sendmail
config.action_mailer.sendmail_settings = {
  location: '/usr/sbin/sendmail',
  arguments: '-i -t -f noreply@yourdomain.com'
}

When sending emails in Swedish, avoid these common triggers:

  • Excessive use of special characters (å, ä, ö)
  • Multiple exclamation points (!!!)
  • Words that might be spam filters' blacklisted in any language

Here's an optimized mailer template:

class UserMailer < ApplicationMailer
  def welcome_email(user)
    @user = user
    headers['X-Priority'] = '3'
    mail(
      to: email_address_with_name(@user.email, @user.name),
      from: email_address_with_name('noreply@yourdomain.com', 'Your App Name'),
      subject: 'Välkommen till vår tjänst'
    )
  end
end

Implement these verification steps:

  1. Use mail-tester.com to analyze your emails
  2. Check your domain's reputation at mxtoolbox.com
  3. Test deliverability with multiple email providers

For enhanced trust, consider Brand Indicators for Message Identification:

# BIMI DNS record
default._bimi.yourdomain.com IN TXT "v=BIMI1; l=https://yourdomain.com/logo.svg;"

Modern spam filters evaluate emails using complex algorithms that analyze technical configurations, content patterns, and sender reputation. For Rails applications using Sendmail, these factors become critical:

# Typical Rails mailer configuration
ActionMailer::Base.smtp_settings = {
  address: 'localhost',
  port: 25,
  domain: 'yourdomain.com',
  user_name: nil,
  password: nil,
  authentication: nil
}

Missing or incorrect DNS records are the most common technical reason for spam flagging. Implement these crucial records:

; SPF Record Example
yourdomain.com. IN TXT "v=spf1 a mx ~all"

; DKIM Record (varies by provider)
mail._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=YOUR_PUBLIC_KEY"

; DMARC Policy
_dmarc.yourdomain.com. IN TXT "v=DMARC1; p=none; rua=mailto:postmaster@yourdomain.com"

Language-specific considerations for Swedish content:

  • Avoid excessive use of special characters (å, ä, ö)
  • Maintain proper HTML-to-text ratio (don't send HTML-only emails)
  • Include unsubscribe links in Swedish ("Avbryt prenumeration")

Modify your /etc/mail/sendmail.mc to include:

dnl Set the From header domain
MASQUERADE_AS(yourdomain.com')dnl
FEATURE(masquerade_envelope')dnl

dnl Enable connection rate limiting
define(confCONNECTION_RATE_THROTTLE', 5')dnl
define(confMAX_DAEMON_CHILDREN', 20')dnl

Implement these in your mailer classes:

class UserMailer < ApplicationMailer
  default from: '"Service Name" ',
          reply_to: 'support@yourdomain.com',
          'List-Unsubscribe' => ''

  def welcome_email(user)
    @user = user
    headers 'X-Priority' => '1',
            'X-Mailer' => 'Ruby on Rails'
    mail(to: @user.email,
         subject: I18n.t('mailers.welcome_subject', locale: :sv))
  end
end

Use these tools to verify your configuration:

  1. MXToolbox for DNS record validation
  2. Mail-Tester for spam score analysis
  3. Gmail's "Show Original" feature to inspect headers