Cold email deliverability for agent-run outbound

The risk with agent-run outbound is not bad writing. It is speed. An agent can draft hundreds of emails in a session, and a mailbox that sends them all at once looks exactly like spam. Deliverability is the part of the system that slows the agent down to the pace inbox providers trust.

1. Send from a separate domain

Keep cold outreach off your main company domain, so a bad week never touches the mail your customers and team depend on. Use a closely related domain, and set up SPF, DKIM and DMARC on it before the first send.

2. Use real mailboxes

Send from real Google Workspace mailboxes, not shared bulk-sending servers. Each email then comes from a real address with a real reply path, and one mailbox’s reputation is not pooled with strangers’. Harp sends from your own mailbox through the Gmail API.

3. Warm up every new mailbox

A new mailbox should start small and grow over weeks. Harp warms each new mailbox over 28 days, from 10 cold emails a day in the first week up to 40 in the fourth, never above the mailbox’s own daily limit.

4. Pace sends and time them for the recipient

Space sends minutes apart rather than in bursts, and cap each mailbox per day. Land emails in the recipient’s working hours, in their timezone. Harp waits a random 2 to 6 minutes between sends from each mailbox, so one mailbox sends 10 to 30 emails an hour at most, however many are due.

5. Send plain text with every email

HTML-only email looks like bulk mail to spam filters. Every email should also carry a plain-text version of the same message. Harp sends both parts with every prospect email.

6. Think twice about tracking

Open pixels and rewritten links are common signals of bulk mail, and open rates are unreliable anyway because mail apps load images on their own. Harp leaves open and click tracking off by default; a workspace can turn it on.

7. Make unsubscribing one click

Major inbox providers expect bulk senders to support one-click unsubscribe through the List-Unsubscribe header (RFC 8058). Harp adds it to cold emails with a signed link per recipient. Unsubscribing adds the address to the suppression list and stops that person’s sequences, and a link scanner opening the page cannot unsubscribe anyone by accident.

8. Stop at the first signal

A reply, a bounce or an unsubscribe should stop that person’s sequence immediately, and suppressed addresses and domains should be checked before every send. Most of the damage in cold email comes from the follow-up that should never have gone out.