What is a Custom Domain?
A custom domain for cold email is a unique domain name separate from your primary business domain, used specifically for outbound email outreach. It protects your main business domain reputation while enabling scalable cold email campaigns.
Custom Domain Examples:
- Primary domain: company.com
- Custom domains: outreach.company.com, company.co, get.company.com
- Protects primary domain from spam complaints
- Isolates cold email risks
- Enables multiple sending identities
- Provides recovery path if reputation damaged
Why Custom Domains Matter
Reputation Protection
Your primary domain is too valuable to risk.
Primary Domain Risks:
- All company communications flow through it
- Blacklisting affects everyone
- Recovery takes months or is impossible
- Business continuity threatened
- Cold email risks isolated
- Primary domain protected
- Business continues if problems occur
- Easy to abandon and replace if damaged
Scalable Outreach
Multiple domains enable volume.
Volume Strategy:
- 1 domain, 1 inbox = 25-50 emails daily
- 10 domains, 30 inboxes = 750-1,500 emails daily
- Spread across domains = better deliverability
Improved Deliverability
Fresh domains with clean reputations.
Deliverability Impact:
- No prior sending history
- No spam complaints
- Clean IP associations
- Warm-up yields consistent high placement
Custom Domain Setup
Domain Selection
Choosing the right domain names.
Best Practices:
- Brand-related: connect.company.com, hello.company.com
- Simpler alternatives: company.co, company.net
- Action-oriented: try.company.com, get.company.com
- Avoid: Numbers, hyphens, misspellings
- Domains registered 1+ years ago
- Faster warm-up and better initial reputation
- More expensive but worth investment
- Verify clean history before purchasing
DNS Configuration
Setting up required records.
Required DNS Records:
- MX Records: Email routing
- SPF Record: Sender authorization
- DKIM Record: Email authentication
- DMARC Record: Authentication policy
- Reverse DNS (PTR): IP reputation
Email Account Setup
Creating inboxes on custom domains.
Setup Process:
- Purchase domain through registrar
- Set up email hosting (Google Workspace, Microsoft 365)
- Configure DNS records
- Create email inboxes (name@customdomain.com)
- Verify authentication
- Begin warm-up
Domain Warm-Up for Custom Domains
Gradual Reputation Building
New domains need warm-up.
Warm-Up Schedule:
- Week 1: 5-10 emails per inbox daily
- Week 2: 15-25 emails per inbox daily
- Week 3: 25-40 emails per inbox daily
- Week 4: 40-50 emails per inbox daily
Warm-Up Best Practices
Effective Warm-Up:
- Send real emails, not empty tests
- Get replies when possible
- Vary email content and timing
- Maintain consistent sending patterns
- Monitor bounce and complaint rates closely
- Manual warm-up services
- Built-in platform warm-up
- Peer-to-peer warm-up networks
Custom Domain Strategy
Multiple Domain Approach
Don't rely on single domain.
Recommended Setup:
- Primary: company.com (never use for cold email)
- Custom #1: outreach.company.com
- Custom #2: connect.company.com
- Custom #3: hello.company.com
- Custom #4-N: get.company.com, try.company.com, etc.
- 2-3 email inboxes per domain
- 25-50 emails per inbox daily maximum
- 50-150 emails per domain daily total
Domain Rotation
Spread volume across domains.
Rotation Strategy:
- Never send 500 emails from single domain daily
- Distribute across 10+ domains
- Randomize sending to avoid patterns
- Monitor each domain's health
Secondary vs. Subdomain
Secondary Domain: company.co
- Completely separate domain
- Best for cold email isolation
- Part of primary domain
- Some reputation risk to primary
- Better brand alignment
Common Domain Mistakes
Using Primary Domain
Never cold email from your main business domain.
Consequences:
- Spam complaints damage business communications
- Blacklisting affects all company emails
- Recovery nearly impossible
- Business continuity risk
Skipping Warm-Up
Sending high volume from new domain immediately.
Result:
- Immediate spam filtering
- Reputation destroyed
- Wasted domain investment
- Poor deliverability forever
Overloading Single Domain
Too much volume from one domain.
Problem:
- One domain sending 500+ emails daily
- Pattern triggers spam filters
- Account suspension likely
- Domain reputation damaged
Ignoring Domain Health
Not monitoring domain reputation.
Consequences:
- Problems discovered too late
- Wasted investment in damaged domains
- Poor results continue unnoticed
- Lead quality and quantity suffer
Domain Health Monitoring
Key Metrics
Track Daily:
- Bounce rate (keep under 2%)
- Spam complaint rate (keep under 0.1%)
- Open and reply rates
- Inbox placement (test with seed accounts)
- Domain reputation scores
Monitoring Tools
Essential Tools:
- Google Postmaster Tools: Gmail deliverability data
- Microsoft SNDS: Outlook/Office 365 deliverability
- MXToolbox: Blacklist and reputation checks
- Mail-Tester: Email content and spam score analysis
- Sender Score: Return Path reputation tracking
When to Abandon Domain
Know when to cut losses.
Abandon Signals:
- Bounce rate consistently above 5%
- Multiple blacklists
- Spam complaints rising
- Open rate consistently below 10%
- Recovery attempts failing
- Add new custom domain
- Migrate good inboxes to new domain
- Abandon damaged domain
- Continue with healthy domains
Key Takeaways
- Custom domain = separate domain for cold email, protecting primary domain
- Use 2-3 custom domains minimum, 10+ for higher volume
- Setup: purchase domain → DNS config (SPF/DKIM/DMARC) → email accounts → warm-up
- Warm-up minimum 21 days: 5-10 → 15-25 → 25-40 → 40-50 emails/day
- 2-3 inboxes per domain, max 25-50 emails per inbox daily
- Avoid: primary domain, skipping warm-up, overloading single domain, ignoring health
- Monitor daily: bounce rates (<2%), spam complaints (<0.1%), inbox placement
- Multiple domains distribute volume and reduce risk
- Tools: Google Postmaster, Microsoft SNDS, MXToolbox, Mail-Tester
- Abandon domain when: bounce rate >5%, multiple blacklists, low open rates
- Aged domains (1+ years) warm faster than new domains
- Secondary domains preferred over subdomains for maximum isolation
- Domain health monitoring essential for sustained deliverability
Sources:
Related Terms
CAC (Customer Acquisition Cost)
Total sales and marketing spend divided by new customers. Lower is better.
Cadence
Sequence and timing of touchpoints in outreach campaign.
Call-to-Action (CTA)
Specific action you want prospect to take. Clear CTA improves conversion.
CAN-SPAM Act
US law regulating commercial email. Requires opt-out mechanism and sender identification.
