Introduction
Setting up a transactional email subdomain (such as tx.yourdomain.com) can feel optional at first, but it becomes very important when incidents happen. Password resets may get delayed during a promo campaign, OTP emails may show as sent while users do not receive them, or you may need to tighten DMARC policies without affecting your entire domain.
This guide is for SaaS and development teams, SMBs, and enterprise IT teams that want a clean and repeatable setup. It explains how to choose a transactional subdomain, which DNS records matter (SPF, DKIM, and DMARC), and how to verify and monitor outcomes using logs and event tracking.
The goal is not a guaranteed inbox result. The goal is cleaner separation, clearer identity signals, and faster troubleshooting as you scale. To get started, review the MailCub Documentation and set up your sending flow in a controlled way.
Quick Answer
- Use tx.yourdomain.com to isolate transactional identity from marketing email and reduce cross-impact.
- Authenticate the subdomain with SPF and DKIM, then add and roll out DMARC safely.
- Keep From addresses stable (for example, security@tx..., billing@tx...) so users and support teams recognize them.
- Verify the domain in your provider dashboard and allow time for DNS propagation (the provided content notes up to 48 hours).
- Use logs and event tracking or webhooks to debug delivered, bounced, and complaint outcomes instead of guessing.
Why It Matters
Transactional email is part of your product reliability. If OTPs or password resets fail, users do not blame email systems. They blame your app.
A transactional subdomain gives you clearer boundaries. It helps separate traffic types, standardize authentication, and narrow down root causes using logs and event signals. In simple terms, it makes “what changed?” much easier to answer when something goes wrong.
Transactional Email Subdomain Naming Strategy
Choose a subdomain that is easy to document, stable, and separate from marketing traffic.
A good transactional subdomain should be:
- Easy to document: tx., notify., or mail.
- Stable: avoid renaming it after launch
- Separate from marketing: keep promo/newsletter traffic on a different subdomain (for example, news. or promo.)
A recommended default is tx.yourdomain.com for product-critical email flows.
Step-by-Step Setup
Step 1: Decide what will send from the subdomain
Use the transactional subdomain for product-critical email only, such as:
- OTP and login codes
- Password resets
- Security alerts
- Invoices and receipts
- Onboarding and account notifications
Avoid mixing marketing newsletters into the same identity if possible.
Step 2: Standardize From addresses on the subdomain
Keep From addresses predictable and role-based. For example:
- security@tx.yourdomain.com for OTP and security messages
- billing@tx.yourdomain.com for invoices
- notifications@tx.yourdomain.com for product updates
The provided content also notes that consistent From addresses are recommended in MailCub deliverability best practices. You can review these setup practices in the MailCub Documentation.
Transactional Email Subdomain DNS Checklist (SPF, DKIM, DMARC)
This is the core setup. Do not invent DNS record values. Always use the exact values generated by your provider.
Step 3: Add SPF for tx.yourdomain.com
SPF authorizes which systems can send on behalf of the subdomain.
Keep SPF simple and clean:
- Publish one SPF record for the sending subdomain
- Remove unused senders
- Avoid too many includes and unnecessary complexity
Step 4: Enable DKIM signing for the subdomain
DKIM helps prove message integrity and helps receivers validate your identity.
The provided content says that in MailCub, “Add Your Domain” includes verifying ownership by setting DNS records (MX, SPF, and DKIM) and then waiting for propagation, which may take up to 48 hours. Follow the exact record values shown in the MailCub Documentation.
Step 5: Add DMARC and roll out safely
Start with monitoring mode before enforcing DMARC. DMARC is most useful after SPF and DKIM are consistently passing and aligned.
If you enforce too early, you may block legitimate mail streams you forgot to account for, such as support tools, billing systems, or older servers.
Step 6: Verify the domain and send a controlled test
Most email APIs reject sends if the From domain is not verified. The provided content notes these MailCub response codes:
- 403 Domain not verified
- 404 Domain not found
The provided content also mentions a MailCub send endpoint and header authentication format shown in MailCub materials. Use the MailCub Documentation to confirm the current integration steps, and review the Transactional Email page for product capabilities.
Step 7: Turn on logs and webhook/event tracking
The provided content highlights that the MailCub Transactional Email page includes real-time event tracking and webhook support. It also mentions Analytics/Logs guidance in MailCub Documentation, including monitoring bounce and complaint rates and API rate limits.
This visibility helps you answer common troubleshooting questions with evidence:
- Was the message queued?
- Was it rejected because the domain was not verified?
- Was it rate limited?
- Did it bounce?
- Was there a complaint?
Transactional Subdomain Launch Checklist
| Area | What to verify | What “good” looks like |
|---|---|---|
| Subdomain choice | tx. is reserved for transactional only | Marketing uses a different subdomain |
| From identity | Stable From addresses on the subdomain | Security, billing, and notifications addresses stay consistent |
| DNS auth | SPF and DKIM published for the subdomain | Verified status appears in the dashboard |
| DMARC rollout | Monitoring first, enforcement later | No unexpected blocks |
| API readiness | API key created and stored securely | No 401 errors and keys are not exposed in frontend code |
| Verification errors | Team understands 403 and 404 | From domain is verified and associated |
| Observability | Logs and events/webhooks enabled | Messages can be traced quickly |
Common Mistakes
- Setting SPF and DKIM for the root domain but forgetting the transactional subdomain
- Changing From domains frequently, which makes debugging and trust harder
- Enforcing DMARC before identifying all legitimate senders
- Testing once and then sending to all users without monitoring logs and events
- Treating subdomains like a deliverability trick instead of an engineering control
Troubleshooting
Issue: API request fails with domain errors
If the From domain is not verified or associated, the provided content says MailCub Documentation lists:
- 403 Domain not verified
- 404 Domain not found
Fix this in order:
- Add the domain in the dashboard
- Complete SPF and DKIM DNS verification
- Wait for propagation (the provided content notes up to 48 hours)
Issue: “Delivered” but user says it did not arrive
Treat this as a visibility and tracing task, not guesswork.
- Check send status in logs and events
- Check bounce and complaint indicators
- Ask for the time and recipient address so you can trace the correct message
FAQ
What is a transactional email subdomain?
A transactional email subdomain is a dedicated subdomain (like tx.yourdomain.com) used only for product-critical email so identity and troubleshooting stay clean.
Should I use tx.yourdomain.com or notify.yourdomain.com?
Either is fine. Pick one naming convention and keep it stable so your DNS, authentication, and monitoring remain predictable.
Do I need SPF and DKIM for the subdomain, or only for the main domain?
You should authenticate the domain you actually send from. If you send from tx.yourdomain.com, configure SPF and DKIM for that subdomain too.
How long does domain verification take?
It depends on DNS propagation. The provided content notes that MailCub Documentation says propagation can take up to 48 hours.
Why is the API returning “Domain not verified” or “Domain not found”?
The provided content says MailCub Documentation describes these as 403 (not verified) and 404 (not associated). The fix is to add and verify the domain in your account and publish the required DNS records.
When should I consider dedicated infrastructure instead of just a subdomain?
Consider dedicated infrastructure when you need strict isolation, advanced compliance or control requirements, or infrastructure-level customization. A subdomain helps with identity separation, but it does not replace dedicated architecture.
Conclusion
A transactional email subdomain is a simple and useful engineering control. It isolates identity, keeps authentication cleaner, and makes debugging easier when users report missing emails. Set it up once, verify it properly, and then rely on logs and event tracking to make decisions using real evidence.
To move forward, review setup and verification steps in the MailCub Documentation, explore real-time tracking and webhook support on the Transactional Email page, and check MailCub Pricing if you are planning implementation capacity or rollout stages.