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Email Service ​

DifficultyAdvanced
Team Size2-3 people
Time~35-40 hours
Demo-ready byStep 5
PrerequisitesNode.js, TCP/IP basics, DNS fundamentals
Built byGmail, ProtonMail, Fastmail, Postfix

Skills you'll earn: SMTP protocol, IMAP implementation, TLS encryption, DNS records (MX/SPF/DKIM), spam filtering

Start with sending a string from one place to another. End with a self-hosted email system.

(Assumes basic Node.js knowledge; networking concepts learned along the way)

Step 1: Send a message between two scripts (~1 hour) ​

Write two Node.js scripts. One listens on a TCP port. The other connects and sends a string. The listener prints it.

  • net.createServer() on one side
  • net.connect() on the other
  • Send "Hello" and see it arrive

You now have: The most primitive message delivery system.

Step 2: Follow the SMTP protocol (~3-4 hours) ​

Your TCP chat works, but no email client can talk to it. Email uses SMTP.

  • Read RFC 5321 (just the basics: HELO, MAIL FROM, RCPT TO, DATA, QUIT)
  • Modify your server to respond with the correct SMTP status codes
  • Use telnet localhost 25 to manually send an email to your server

You now have: An SMTP server that can receive mail.

Step 3: Store received emails (~2 hours) ​

Your server receives messages but they vanish after printing.

  • Save each email to a file: one file per message
  • Include headers (From, To, Subject, Date) and body
  • Parse the raw SMTP DATA into structured fields

You now have: A mailbox.

Step 4: Send emails out (~3-4 hours) ​

You can receive but not send. Time to be an SMTP client.

  • Connect to another SMTP server (like Gmail's) and deliver a message
  • Handle DNS MX record lookups to find the right server for a domain
  • Add proper headers so the receiving server doesn't reject you

You now have: Bidirectional email.

Step 5: Add a web interface (~3-4 hours) ​

Reading email from files is painful.

  • Build a simple web UI: inbox list, click to read
  • Show From, Subject, Date in the list
  • Show full message body when clicked
  • Add a "Compose" form that sends via your SMTP client

You now have: Webmail.

Step 6: User accounts and mailboxes (~3 hours) ​

Everyone's mail is in one pile.

  • Add user registration and login
  • Each user gets their own mailbox directory
  • Route incoming mail to the correct user based on the RCPT TO address

You now have: Multi-user email.

Step 7: IMAP support (~4-5 hours) ​

Your users want to use Thunderbird or Apple Mail, not just your web UI.

  • Implement basic IMAP (RFC 3501) — LOGIN, SELECT, FETCH, SEARCH
  • This lets desktop/mobile email clients connect to your server
  • Start with read-only access, then add delete and move

You now have: A real email server.

Step 8: DNS records (~2-3 hours) ​

Emails you send get rejected or land in spam.

  • Set up MX records so other servers know where to deliver mail for your domain
  • Add SPF records (which IPs are allowed to send for your domain)
  • Add DKIM (cryptographic signature on outgoing mail)
  • Add DMARC policy

You now have: Deliverable email.

Step 9: TLS encryption (~2 hours) ​

Everything is in plaintext. Anyone on the network can read your emails.

  • Add STARTTLS to your SMTP server
  • Generate or obtain TLS certificates
  • Enforce encryption for connections

Step 10: Spam filtering (~3 hours) ​

Your inbox is filling with junk.

  • Build a basic spam filter: check SPF/DKIM of incoming mail
  • Add a blocklist of known spam domains
  • Score messages based on content heuristics
  • Move suspected spam to a separate folder

Step 11: Attachments (~2-3 hours) ​

  • Parse MIME multipart messages
  • Extract and store attachments
  • Display them in the web UI
  • Allow attaching files when composing

Step 12: Search (~2 hours) ​

  • Index messages for full-text search
  • Search by sender, subject, body, date range

Step 13: Deploy (~3 hours) ​

  • Containerize the whole thing
  • Deploy to the cluster
  • Open ports 25 (SMTP), 143 (IMAP), 443 (webmail)
  • Monitor delivery rates and bounce handling

Useful Resources ​

Where to go from here ​

  • Calendar integration (CalDAV)
  • Contacts (CardDAV)
  • Mailing lists
  • Encryption (PGP/S-MIME)
  • Admin dashboard with usage stats