Under the Hood: How MailGuard Routing Works

A technical deep dive into the architecture of private email forwarding. How we hide your identity even when you reply.

Under the Hood: How MailGuard Routing Works

MailGuard isn't just a forwarder. It's an intelligent firewall for your inbox. Traditional forwarding is simple: A -> B. Privacy-preserving forwarding with Reply Masking ("Magic Reply") is a complex orchestration of cryptography, temporary tokens, and edge computing.

This article peels back the layers to show how we process emails in milliseconds without ever storing your data on a disk.

The One-Way Trip (Inbound)

When someone sends an email to your alias (vendor@your-domain.com), the goal is to get it to your real inbox (real@gmail.com) while stripping away tracking pixels and analyzing for threats.

The "Magic" Reply-To Header

The most critical step above is Rewriting the Reply-To. We don't just pass the sender's email (bad-guy@spam.com) as the reply address. If we did, when you hit "Reply," you would be emailing them directly from your real Gmail, exposing your identity.

Instead, we generate a cryptographic token. Original Reply-To: bad-guy@spam.com Rewritten Reply-To: reply+<encrypted_token>@mailguard-system.com

This token contains:

  • The original sender's address.
  • The alias used (vendor@).
  • An expiration timestamp.
  • A signature to prevent tampering.

The Return Trip (Outbound / Magic Reply)

When you hit "Reply" in Gmail, you are technically emailing the MailGuard System, not the stranger. This allows us to intercept the message and swap identities.

  1. Ingestion: You execute a reply. To Google, you are emailing mailguard-system.com.
  2. Decryption: Our worker receives reply+<token>. It uses our private key to decrypt the payload.
  3. Masquerading: We reconstruct the email.
    • From: vendor@your-domain.com (Your Alias)
    • To: bad-guy@spam.com (Original Sender)
  4. Authentication: This is the hard part. To send as your-domain.com, we must satisfy SPF and DKIM. Because you added our records to your DNS during setup (see our Setup Guide), we are authorized to cryptographically sign this message.
  5. Delivery: The bad guy receives a reply from your alias. They have no way of knowing your real Gmail address even exists.

Privacy By Design: Ephemeral Processing

A core tenet of MailGuard is Zero Persistence.

  • We do not store the body of your emails in a database.
  • We process the text stream in memory (RAM) at the Edge (Cloudflare Workers).
  • Once the forward command is issued, the RAM is cleared.

The only things we store in our database are metadata for your dashboard:

  • "Email received for alias X at [Time]."
  • "Email blocked for alias Y (Reason: Rule)."

This architecture ensures that even if our database were leaked, your private conversations would remain private, because they simply aren't there.