Introduction
If your app sends OTPs, password resets, receipts, and security alerts from your main domain, even a small change can create a large blast radius. A marketing spike, a DNS update, or a new sender can make “sent but not received” issues much harder to isolate and fix.
This guide explains how to move transactional email off your main domain using a dedicated subdomain such as tx.yourdomain.com and a staged rollout plan. It is written for SaaS and development teams, SMBs, and enterprise IT teams that want a migration process that protects login flows and reduces risk.
You will learn how to set up SPF, DKIM, and DMARC on the new subdomain, verify it in your email provider, move traffic in waves, and use logs and event tracking to validate outcomes. DNS propagation can take time, and the provided documentation notes it may take up to 48 hours, so the focus here is safe sequencing and evidence-based troubleshooting.
Before you start, use the Mailcub documentation to review the domain verification process and confirm your setup steps.
Quick Answer
- Create a dedicated transactional subdomain (for example, tx.yourdomain.com) and keep marketing email separate.
- Authenticate the subdomain with SPF and DKIM first, then roll out DMARC safely.
- Verify the subdomain in your provider dashboard and allow time for DNS propagation.
- Migrate in phases: low-risk first, medium-risk next, and login-critical flows last.
- Use logs and webhook or event tracking to confirm delivered, bounced, and deferred outcomes.
Why This Matters
Transactional email is part of your product reliability. If users cannot receive a reset link or OTP, the experience fails immediately.
Moving transactional mail to a dedicated subdomain reduces cross-impact from other sending activity. It also makes investigations faster because your logs and event signals are tied to one identity instead of all mail sent from the same domain.
The provided content also highlights MailCub best practices such as consistent From addresses, authentication setup, bounce and complaint monitoring, and log visibility. These practices become much easier to manage once transactional traffic is isolated.
How to Move Transactional Email Off Your Main Domain Without Breaking Login Flows
Treat this as a production migration. The safest approach is to isolate identity first, then move traffic gradually with a rollback plan ready.
Before changing application code, decide:
- The transactional subdomain name (such as tx. or notify.)
- The rollout order for email types
Use a Phased Rollout Plan
A phased rollout helps keep risk controlled. Do not move OTP and password reset messages on day one.
Plan your migration in waves:
- Wave 1: low-risk notifications
- Wave 2: receipts, invoices, and invites
- Wave 3: OTP, password reset, and security alerts
Step-by-Step Migration Plan
Step 1: Inventory email types and mark login-critical flows
List all transactional templates and tag each one as low, medium, or high risk. Keep the list simple.
If a template affects login or account recovery, treat it as high risk and move it in the final wave.
Step 2: Choose one transactional subdomain and keep it stable
Good default options include:
- tx.yourdomain.com
- notify.yourdomain.com
Choose one naming pattern and keep it stable. Renaming later creates new identity signals and forces re-verification and re-testing.
Step 3: Standardize From addresses on the new subdomain
Use clear, role-based From addresses on the transactional subdomain, such as:
- security@tx...
- billing@tx...
- notifications@tx...
The provided content notes that MailCub docs recommend consistent From addresses as part of deliverability best practices.
Step 4: Add the subdomain in your provider and publish SPF/DKIM
According to the provided content, MailCub documentation describes this flow:
- Add your domain in the dashboard
- Verify ownership using DNS records (MX, SPF, DKIM)
- Wait for DNS propagation (up to 48 hours)
Do not guess DNS values. Use the exact records generated by your provider. You can confirm the setup steps in the Mailcub docs.
Step 5: Add DMARC safely (monitor first)
Start with monitoring mode for DMARC. Only move toward enforcement after SPF and DKIM are passing consistently for the new transactional identity.
This reduces the chance of blocking legitimate mail from older systems or unknown senders during the migration.
Step 6: Route a small percentage of low-risk mail first
Update your app configuration so only low-risk notifications send from the new subdomain at first.
Run this for a short period, review logs and events, and then expand gradually. Keep rollback ready so you can switch back to the old identity quickly if needed.
Step 7: Validate with API responses and logs
During migration, treat API responses as setup signals, not application bugs. The provided content lists these MailCub response codes as important for migration checks:
- 403 Domain not verified
- 404 Domain not found
- 429 Rate limit or quota exceeded
The provided content also notes that MailCub’s product page shows the send endpoint and header authentication format. You can review the Transactional Email product page while validating your send flow.
Step 8: Turn on event tracking and webhooks
The provided content highlights MailCub real-time event tracking and webhook support. Use these features during migration so you can monitor outcomes with evidence.
Track:
- Delivered vs bounced vs deferred
- Complaints
- Throttling and rate-limit events
- Trend changes as each wave is moved
Step 9: Move medium-risk, then high-risk flows
After Wave 1 is stable, move Wave 2. Only move Wave 3 after you have confirmed stable outcomes and clean monitoring signals.
For OTP and password reset flows:
- Test across major mailbox providers your users commonly use
- Keep a clear incident playbook ready for support and engineering teams
Migration Checklist (Gate Before the Next Wave)
| Gate | What you check | Pass criteria |
|---|---|---|
| Subdomain verification | SPF and DKIM are published and verified | No 403 or 404 errors in testing |
| Sending configuration | From addresses use the new identity | Consistent role-based From addresses |
| Observability | Logs and event tracking are available | You can trace a message by ID and status |
| Rate control | You will not overload the API | Batching and backoff are planned; 429 is monitored |
| Rollback | Old identity still works | Critical flows can be reverted quickly |
| Wave stability | Error rates are stable | No new spikes before expanding |
Common Mistakes
- Migrating OTP and password reset flows first instead of proving the new identity with low-risk mail
- Publishing SPF and DKIM for the root domain but not for the transactional subdomain
- Enforcing DMARC before all legitimate senders are verified
- Ignoring 403 and 404 errors and spending time debugging application code instead
- Shipping the migration without logs or event monitoring
Troubleshooting
Problem: 403 “Domain not verified”
The provided content states that MailCub docs describe this error and the related verification flow.
Fix it in this order:
- Add the exact subdomain in the dashboard
- Publish the required SPF and DKIM DNS records
- Wait for DNS propagation (up to 48 hours)
Problem: “Delivered” but the user did not receive the email
Start with logs and events before making assumptions:
- Confirm the status and timestamp
- Check bounce and complaint indicators
- Verify the message was sent from the correct transactional subdomain identity
FAQ
What does it mean to move transactional email off your main domain?
It means sending product-critical emails such as OTPs, resets, and receipts from a dedicated subdomain instead of your root domain. This makes authentication and monitoring more isolated and easier to manage.
Should I use tx.yourdomain.com or notify.yourdomain.com?
Either option works. Choose one naming convention and keep it stable so DNS authentication, monitoring, and troubleshooting stay predictable.
Do I need SPF/DKIM/DMARC for the subdomain or only the main domain?
You should authenticate the domain you actually send from. If your From address uses a subdomain, configure SPF and DKIM for that subdomain, then roll out DMARC safely after authentication is consistently passing.
How long does domain verification take?
It depends on DNS propagation. The provided content notes that MailCub documentation says it can take up to 48 hours after publishing the required DNS records.
What order should I migrate email types to avoid outages?
Migrate in waves: low-risk notifications first, then receipts and invites, and move OTP and password resets last. This reduces the chance of breaking login flows while the new identity stabilizes.
How do I troubleshoot 403 “Domain not verified” and 404 “Domain not found”?
The provided content notes that MailCub docs list 403 as Domain not verified and 404 as Domain not found. Add the exact subdomain in your account, publish SPF and DKIM records, wait for propagation, and retry.
Conclusion
To move transactional email off your main domain safely, isolate identity with a dedicated subdomain, authenticate it properly, and migrate in waves while monitoring logs and event signals. This reduces cross-impact, makes incidents easier to debug, and helps protect login-critical flows during changes.
Use the Mailcub documentation to complete setup steps, validate your sending flow with the Transactional Email product, and review the pricing page if you are planning your rollout and capacity.