Phase 14Days 136-139

Custom Domains & Transactional Email

DNS from scratch, Hostinger → Vercel, and email that reaches inboxes

Phase Goal

Understand what a domain and DNS records actually do, connect a Hostinger domain to Vercel with confidence, and send authenticated transactional email from a real domain.

Progress

Notes

Open the written lectures for this course before checking off the phase topics.

Projects

Multilingual Clinic Appointment Confirmation — DEPLOYED

Day 136: Domains & DNS — From First Principles

Domain name vs URL vs IP address: three different jobsRegistrar, DNS host, web host, and email provider: who controls whatHow a browser resolves example.com before it can request your appWhy buying a domain from Hostinger does not automatically host a website there

A and AAAA records: point a name at an IPv4 or IPv6 addressCNAME records: make one hostname alias another hostnameTXT records: publish ownership proofs and email policiesMX records: tell senders which servers accept mail for a domain

Apex/root domain: yourdomain.com; www is a separate hostnameSubdomains such as app., api., mail., and staging. are independent namesChoose one canonical public URL and redirect the alternativesWhy a CNAME works naturally for www but can be limited at the apex

TTL is a cache lifetime, not a timer that makes records instantly changeWhy resolver caches make different networks appear to disagreeUse dig/nslookup and provider verification screens to inspect recordsNever delete an unfamiliar MX or TXT record before understanding its owner

Day 137: Custom Domain on Vercel — Hostinger Walkthrough

Deploy the project to Vercel first and confirm its generated vercel.app URL worksPick the production hostname before touching DNS: root domain or wwwKeep access to the Hostinger domain/DNS dashboard and Vercel project settingsRecord existing DNS entries so a mistake is reversible

Project Settings → Domains: add yourdomain.com and inspect Vercel’s required recordsUnderstand Vercel’s verification status and why it refuses conflicting recordsSet the preferred domain and configure a redirect for the other hostnameUse Vercel’s current record values instead of copying stale values from a tutorial

Open Hostinger hPanel → Domains → DNS Zone Editor for the purchased domainCreate the A record for @ and CNAME for www exactly as Vercel currently requestsRemove only records that conflict with those exact hostnames; leave mail records aloneReturn to Vercel to verify, then wait and recheck rather than repeatedly changing records

Vercel provisions TLS after DNS points correctly; do not buy a separate SSL certificateTest root, www, HTTPS, a deep route, and a mobile device after verificationCheck canonical redirects so search engines and users see one URLKnow the common failure modes: wrong host field, duplicate records, stale nameservers, and cache delay

Day 138: Transactional Email & Domain Authentication

User-triggered messages: receipts, password resets, verification, and account noticesWhy app code should call a provider API instead of running its own mail serverChoose a provider such as Resend, Postmark, or SES based on delivery needs and costKeep provider keys server-side and never expose them in browser JavaScript

From address, reply-to address, and return path: what each communicatesWhy hello@yourdomain.com earns more trust than a random shared senderSeparate website and email concerns with deliberate hostnames such as mail.yourdomain.comCreate a verified sending domain in the provider before attempting production sends

SPF TXT record: which services are allowed to send for your domainDKIM CNAME/TXT record: how receivers verify an email signatureDMARC TXT record: require alignment and choose a cautious p=none rollout firstAdd the provider-generated records in Hostinger without replacing unrelated DNS entries

Send a server-side welcome email with a provider SDK and environment variableInclude a plain-text alternative, meaningful subject, and working reply-to addressUse provider logs to distinguish accepted, delivered, bounced, and failed messagesTest a real inbox and spam folder before declaring email finished

Day 139: Project — Multilingual Clinic Appointment Confirmation

Use a simple HTML email or react-email components for maintainable templatesPrefer a narrow layout, inline-safe styling, descriptive links, and image alt textPass only trusted, validated data into the templateBuild one reusable layout and a welcome or contact-confirmation message

Validate recipient input on the server, even if the form validates in the browserRate-limit public contact forms and never turn them into an open relayHandle provider failures with a useful UI state and server logsDo not send email before the primary user action is safely stored

Deploy a small clinic-request app to Vercel and connect a Hostinger domainChoose and enforce the canonical root/www URL with HTTPS workingAuthenticate the domain with your email provider using SPF, DKIM, and starter DMARCShip one transactional flow: welcome, contact confirmation, or password-reset request

Capstone

Multilingual Clinic Appointment Confirmation — DEPLOYED

Launch a clinic confirmation flow on a Hostinger domain connected to Vercel: a patient requests an appointment, receives an authenticated language-appropriate confirmation from the custom domain, and follows a short-lived reschedule link. Document DNS, SPF, DKIM, DMARC, delivery diagnostics, privacy boundaries, and rollback.

Hostinger → Vercel canonical HTTPS domain with a documented DNS record table.
Verified From domain with SPF, DKIM, starter DMARC, and provider delivery logs.
One safe server-side email flow with validated input, rate limits, and a secure status link.

Write a README table: record purpose, hostname, value source, and owning providerStore secrets in Vercel environment variables, never in source controlCapture screenshots of Vercel verification and provider domain verificationState the rollback plan: which record to restore and how to return to the vercel.app URL

Phase Complete!

After this phase, you'll be able to:

Explain domains, DNS, A/AAAA, CNAME, TXT, MX, TTL, and propagation from first principles
Connect a Hostinger-managed domain to Vercel and configure a canonical HTTPS URL
Authenticate transactional email with provider-managed SPF, DKIM, and starter DMARC records
Ship one safe server-side email flow from a real From domain

You can put a real product on its own domain and make its essential email arrive credibly — without treating DNS as copy-paste magic.

Back to all phases

Custom Domains & Transactional Email

Phase Goal

Day 136: Domains & DNS — From First Principles

The internet’s address book

DNS records you will actually use

Root, www, and subdomains

TTL, propagation, and safe debugging

Day 137: Custom Domain on Vercel — Hostinger Walkthrough

Prepare the app and domain

Add the domain in Vercel

Tutorial: enter the records in Hostinger

HTTPS and launch checks

Day 138: Transactional Email & Domain Authentication

What transactional email is for

Use a real From domain

SPF, DKIM, and DMARC without magic

First trustworthy send

Day 139: Project — Multilingual Clinic Appointment Confirmation

Email content that works everywhere

A safe send endpoint

Build and verify the application update flow

Multilingual Clinic Appointment Confirmation — DEPLOYED

Document the setup for future you

Phase Complete!