Deliverability

Whether a message lands in the inbox is decided by the receiving provider, per message, from the sender’s reputation, authentication, the content and how recipients react. We handle the infrastructure (our own mail server, DKIM signing, reverse DNS, blocklist monitoring, bounce handling). What your agents send, to whom and how often is up to you, and it matters more.

#Inbox, Promotions, spam

• Spam means the provider doesn’t trust the sender or the message: failed authentication, a poor reputation, many complaints or bounces, or content that looks like fraud.
• Promotions (and similar tabs) is not spam. It is where a provider files mail that looks like marketing: heavy HTML layouts, many images and links, tracking, discount language, sent to many people at once. It is a sorting decision, and you can’t switch it off.
• Inbox is what a personal, one-to-one message gets: plain wording, few links, a reply to something the recipient wrote, from an address they have exchanged mail with before.

Agents that write one-to-one mail (support replies, scheduling, sign-ups, follow-ups) should look like a person writing: short plain text, or light HTML with a text part, one or two links, no tracking. That is also what we send: every message has a text/plain part, and we add no footer, tracking pixel or link rewriting.

#Authentication and alignment

Receivers check three things: SPF (is this server allowed to send for the envelope domain?), DKIM (is the message signed by a key published in DNS?) and DMARC (does one of them pass for the domain in the visible From address, and what should happen if not?). DMARC needs alignment: the domain that passes SPF or DKIM must match the From domain (or share its organizational domain).

• On our shared domain (agents.agentboxd.com) everything is set up for you: SPF, DKIM with our key, and DMARC all align.
• On your own domain, connect it and publish the records we show. We sign with your domain’s own DKIM key, so DKIM aligns with your From address, and the SPF include:agents.agentboxd.com covers our server. Start DMARC at p=none, read the reports for a few weeks, then move to p=quarantine and p=reject.
• Use a subdomain such as mail.yourcompany.com for agents, so their reputation is separate from your people’s mail on the main domain.
• Rotate the DKIM key yearly (how); the domain page reminds you.

#Warm-up

Providers are suspicious of a new domain or address that suddenly sends a lot. Build volume over a few weeks:

• Week 1: tens of messages a day, to people who expect them (colleagues, test accounts, customers who wrote first).
• Then roughly double the daily volume every few days while bounces stay under 2 % and complaints near zero.
• Prefer replies over cold mail. A conversation the recipient started is the strongest positive signal there is.
• Keep volume steady. Spikes (a whole list in one hour) look like a compromised account.

#Content and recipients

• Only write to people who expect it. Never buy or scrape lists; a hard bounce or a spam complaint costs far more than one message gains.
• Stop at the first bounce. Hard-bounced addresses are suppressed for every workspace (sends to them answer 422 recipient_suppressed), and a complaint suppresses the address too.
• Honour "stop" and unsubscribe requests at once, and make them easy: a line such as “reply STOP and we won’t write again” works for one-to-one mail.
• Write a real subject and a greeting, keep links few and to your own domain, avoid URL shorteners and attachments nobody asked for.
• Give the inbox a clear display name (“Acme Support”) and keep the same address for the same purpose.
• Watch for replies: an agent that answers out-of-office messages or other agents can loop. Check Auto-Submitted headers and the ai:auto-reply label, and use pause as a kill switch.

#Our limits and why

Every workspace shares our sending IP and, unless it uses its own domain, our sending domain. One workspace’s spam would land everybody else’s mail in spam, so we enforce limits before providers do.

A suspended workspace can still receive and read mail. Clean up the recipient list and write to abuse@agentboxd.com to lift it. See Plans and limits for every limit error.

#The deliverability API

GET /v1/deliverability (permission metrics:read, workspace keys only) summarises what affects your mail: your bounce and complaint rates over 7 and 30 days next to the suspension thresholds, how many of your contacts are suppressed, the SPF, DKIM and DMARC status of your domains, and whether our shared sending IP is on a blocklist.

curl https://api.agentboxd.com/v1/deliverability \
  -H "Authorization: Bearer $AGENTBOXD_API_KEY"

{
  "rates": {
    "last_7_days":  { "sent": 412, "bounced": 3, "complained": 0, "bounce_rate": 0.0073, "complaint_rate": 0 },
    "last_30_days": { "sent": 1630, "bounced": 11, "complained": 1, "bounce_rate": 0.0067, "complaint_rate": 0.0006 }
  },
  "thresholds": { "bounce_rate": 0.05, "complaint_rate": 0.001, "min_sent": 20 },
  "suppressed_contacts": 9,
  "domains": [
    { "id": "…", "domain": "mail.acme.com", "status": "verified", "spf": "ok", "dkim": "ok", "dmarc": "ok", "dkim_rotation_due": false }
  ],
  "shared": {
    "sending_domain": "agents.agentboxd.com",
    "ip_reputation": {
      "status": "clean",
      "checked_at": "2026-09-25T09:00:00.000Z",
      "lists": [
        { "name": "Spamhaus ZEN", "status": "clean" },
        { "name": "Barracuda", "status": "clean" },
        { "name": "SpamCop", "status": "clean" },
        { "name": "UCEPROTECT L1", "status": "clean" }
      ]
    }
  }
}

We check our IP once an hour on Spamhaus ZEN, Barracuda, SpamCop and UCEPROTECT Level 1, and we are alerted when it is listed or when bounce or complaint rates across all workspaces rise. The TypeScript SDK has mr.deliverability(), Python client.deliverability(), and MCP clients get_deliverability.