The domain you send emails from determines whether you land in the inbox or spam folder. Most companies get this wrong. They create clever-sounding domains for campaigns or stick everything on one subdomain. Both approaches can hurt deliverability.
This guide walks you through organizing your sending domains properly, following M³AAWG guidelines that Gmail, Outlook, and other major providers use to evaluate your emails.
TL;DR: Use Subdomains, Not Cousin Domains
If your company is example.com, send from news.example.com, offers.example.com, or support.example.com. Never create "cousin domains" like example-deals.com or get-example-offers.com. They look fake to spam filters and destroy your deliverability.
Understanding Domain Trust
Email providers judge domains the same way people judge storefronts. A Louis Vuitton store in a luxury mall gets instant trust. A "Luis Vutton Deals" shop in a strip mall gets suspicious looks.
When recipients see offers.nike.com in their inbox, they recognize Nike immediately. Nike-super-deals.com triggers alarm bells. People make this judgment in under a second. Spam filters work exactly the same way.
The choice between using a subdomain of your main domain versus creating a separate "cousin" domain determines your email fate. One builds trust. The other destroys it.
Cousin Domains Kill Deliverability
Cousin domains are separate domains that try to look related to your brand. Summer-deals-at-example.com. Shop-example-now.com. Example-exclusive-offers.net. Companies register these thinking they're being creative with campaign names.
Recipients and spam filters can't verify these domains belong to you. They match exactly what phishing attacks look like. Most companies forget to redirect these domains to their main site. Someone clicks through and hits a dead page. Everything about it screams scam.
You don't need special domains for seasonal campaigns or promotions. Summer.example.com works better than summer-deals-at-example.com. Shop.example.com beats shop-example-now.com every time. Subdomains give you campaign flexibility without sacrificing trust.
Subdomains: The Professional Approach
Subdomains keep all your email under one trusted umbrella while letting you segment different types of communication. Every successful email program uses this structure.
Marketing emails go through descriptive subdomains: offers.example.com for promotions, news.example.com for newsletters, updates.example.com for product updates. Transactional emails that customers expect use info.example.com for general notifications, orders.example.com for purchase confirmations, account.example.com for password resets.
Regional operations get their own subdomains too. UK.example.com for British customers, eu.example.com for European Union, asia.example.com for Asian markets. Different departments use support.example.com for customer service, sales.example.com for outreach, hr.example.com for internal communications.
Recipients instantly recognize anything ending in example.com as legitimate. Your subdomains inherit trust from your main domain's reputation. You manage everything under one domain registration. Each subdomain builds its own reputation for different content types. The whole system looks professional to everyone involved.
Planning Your Subdomain Structure
Most companies send more types of email than they realize. Take inventory of everything you send. Marketing campaigns including newsletters, promotions, and product launches. Transactional emails covering order confirmations, shipping notifications, and receipts. Account emails for password resets, welcome messages, and profile updates. Support emails containing ticket responses, help articles, and status updates. Don't forget internal communications if you email employees.
Map each email type to a clear subdomain. Keep names short and obvious. News.example.com beats x7news.example.com. Offers.example.com wins over DEALS.example.com. Orders.example.com crushes trans_01.example.com. Help.example.com works better than noreply.example.com, especially since Outlook started penalizing no-reply addresses in 2024.
Email Header Alignment Matters
Your email headers work like business cards. When all your contact information matches across cards, website, and emails, people trust you more. Email authentication works the same way.
Using a subdomain like news.example.com means aligning everything. Your Return-Path shows bounce@news.example.com. The From address displays hello@news.example.com. DKIM Signature contains d=news.example.com. Reply-To directs to feedback@news.example.com. Everything points to the same subdomain, creating perfect alignment that authentication systems love.
You can also keep your main domain in the From address while maintaining organizational alignment. Return-Path uses bounce@bounce.example.com. From displays hello@example.com. DKIM Signature shows d=example.com. Reply-To goes to support@example.com. Everything stays under the example.com umbrella, preserving trust while giving you flexibility.
DNS Configuration Options
Three main approaches exist for setting up subdomain DNS records. Each offers different trade-offs between control and convenience.
Direct setup works when you manage your own DNS. You add SPF records specifying which servers send mail. MX records handle bounces. A records enable web tracking. DKIM keys provide authentication. You control everything and can update instantly. The downside is needing technical knowledge to manage it properly.
CNAME delegation lets your email service provider manage certain records. Bento uses this approach, handling DNS complexity for you. You point subdomains to your ESP's servers using CNAMEs. They manage the technical details. Your ESP can optimize settings without bothering you, though you lose some visibility into what's happening.
NS delegation completely hands off a subdomain to your ESP, though few companies choose this. You delegate the entire subdomain to their nameservers. Zero maintenance required, but you give up all control. Most avoid this unless they have unusual requirements.
CNAME delegation typically offers the best balance. You maintain overall control while your ESP handles the complex stuff.
Volume Guidelines for Subdomains
Each subdomain needs sufficient volume to build reputation without concentrating too much risk in one place.
Low-volume senders under 100,000 emails monthly can use a single subdomain or domain. Simple to manage with enough volume to establish basic reputation.
Medium-volume senders between 100,000 and 1,000,000 monthly should separate marketing from transactional mail. Consider also geographic splits if you serve distinct regions.
High-volume senders over 1,000,000 monthly need dedicated subdomains per email type. Geographic subdomains become essential. Department-specific domains help distribute load. Multiple DKIM selectors prevent bottlenecks.
The optimal range sits between 100,000 and 500,000 emails monthly per subdomain. Below 100,000, reputation never solidifies and delivery stays unpredictable.
Migration Without Disruption
Moving from your main domain or cousin domains to proper subdomains requires careful planning to maintain delivery during transition.
Start with foundation work. Create your subdomain infrastructure. Configure SPF, DKIM, and DMARC for each subdomain. Test with your internal team first. Send to your most engaged 5% of subscribers or activate a few low-volume automations.
Then transition gradually over two weeks. Move transactional emails first since they get better engagement. Ramp volume from 5% to 25% to 50% to 100%. Watch your metrics constantly. Keep old domains active as fallback options.
Finally, complete the switch. Migrate all remaining volume. Set up forwarding from old addresses. Update every template and system. Monitor closely for the first month to catch any issues early.
Mistakes That Hurt Deliverability
Using mail.example.com for everything defeats the purpose of subdomains. You lose segmentation benefits and concentrate reputation risk. Split email types across multiple subdomains instead.
Cryptic names like esp47.example.com or x3n.example.com confuse recipients. People see these in their inbox and immediately get suspicious. Pick clear, descriptive names that make instant sense.
Setting up subdomains then ignoring them causes problems to compound silently. Schedule monthly reviews of each subdomain's performance. Check authentication passes, engagement rates, and spam complaints.
Security for Your Subdomains
Subdomains need the same protection as your main domain. Configure DMARC policies for each one. Monitor for unauthorized usage weekly. Upgrade to 2048-bit DKIM keys. Protect against subdomain takeover attacks. Audit configurations quarterly. Document every active subdomain in a central location.
Implementation Timeline
Start today by auditing your current setup. List every email type you send. Match each to an appropriate subdomain. Takes about 30 minutes.
This week, check subdomain availability with your DNS provider. Decide between direct setup, CNAME delegation, or NS delegation. Coordinate timing with your ESP. Budget 2-3 hours total.
Over two weeks, implement DNS changes methodically. Verify authentication with testing tools. Start your gradual migration. Monitor everything closely.
Essential Tools
Planning requires basic tools. A spreadsheet works fine for mapping subdomain structure. DNS checkers verify your configurations. Authentication validators test SPF, DKIM, and DMARC.
Monitoring needs ongoing attention. Postmaster tools from Gmail and Outlook track subdomain reputation. DMARC reports show authentication success rates. Your ESP provides engagement metrics per subdomain.
Scaling Your Domain Strategy
Your subdomain approach evolves with growth.
Startups with under 50,000 subscribers keep things minimal. One or two subdomains maximum. Focus entirely on authentication basics.
Growing companies from 50,000 to 500,000 subscribers expand strategically. Three to five subdomains organized by purpose. Geographic separation if serving multiple regions.
Enterprise senders over 500,000 subscribers need complete architecture. Department and regional subdomains. Multiple DKIM selectors for load distribution. Dedicated IPs for critical subdomains.
Build on Solid Ground
Your sending domain forms the foundation of your email program. Cousin domains crumble under scrutiny. Subdomains stand strong.
ISPs expect subdomain structures. Recipients recognize and trust them. Major brands all use them. They work consistently, build reputation effectively, and scale with your growth.
Start with three core subdomains. News.yourdomain.com handles marketing. Account.yourdomain.com manages transactional mail. Support.yourdomain.com covers customer service. Expand from there as needed. The foundation matters more than the size.