Reply forwarding

How it works

1. A recipient replies to a message you sent from you@yourdomain.com.
2. Their mail server looks up the MX for yourdomain.com and delivers the reply to whatever it points at.
3. If your MX points at the relay, the relay receives the reply.
4. The relay rewrites the envelope sender (SRS) so SPF doesn't break, then delivers the reply to your configured forward-to mailbox.

Forward-to is configured per domain. Each domain you enroll gets its own forward-to address.

MX setup

Reply forwarding requires an MX record on your sending domain that points at the relay's inbound listener. Add this record:

yourdomain.com  MX  10  smtp.atmos.email.

If you already have an MX (e.g. you receive direct mail at this domain via Fastmail or Gmail), reply forwarding will conflict with it. In that case, use a subdomain just for sending:

news.yourdomain.com  MX  10  smtp.atmos.email.

Then enroll news.yourdomain.com instead. Your apex keeps its existing inbox, and replies to the subdomain flow to your forward-to.

Configuring forward-to

Set the forward-to mailbox at /account under the domain settings, or via the API:

PUT /member/forward-to
Authorization: Bearer atmos_your-api-key
X-Atmos-DID: did:plc:your-did

{
  "domain": "yourdomain.com",
  "forward_to": "you@personalmail.com"
}

The forward-to address can be anywhere. It doesn't need to be at your sending domain.

What happens if forward-to is empty

If you haven't set a forward-to for a domain, the relay rejects inbound mail with 550 address not available. Senders see a bounce. Set a forward-to before pointing your MX at the relay, or skip the MX setup if you don't want to receive replies at all.

Why SRS

When the relay forwards a reply, it has to decide what envelope sender to use. Using the original sender (some-stranger@gmail.com) would break SPF at your forward-to mailbox: Gmail's SPF says only Gmail's IPs can send for gmail.com, and the relay isn't on that list.

SRS (Sender Rewriting Scheme) rewrites the envelope sender to a relay-owned address that encodes the original. SPF checks pass at the destination, and bounces still find their way back to the original sender.

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