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Email deliverability is the art and science of getting your emails into your recipients’ inboxes. It’s not just about hitting “send” — it’s about building a good reputation with email providers like Gmail and Outlook so they trust you’re sending valuable content, not spam. High deliverability is the key to successful email automation. This guide covers both your sending strategy and your email content.

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The technical foundation: SPF, DKIM, and DMARC

Before you send your first email, it’s important to have the right technical foundation in place. SPF, DKIM, and DMARC are DNS records that act as a digital signature, proving to email providers that you are who you say you are.
OpenMail handles all of this automatically for openmail.sh — no setup needed on your end. Just something to know about.
All inboxes use openmail.sh by default. OpenMail handles SPF, DKIM, and DMARC automatically. Need a custom domain like agent@yourdomain.com? Contact us.

Inboxes

Learn about inbox address formats and how to configure your sending domain.

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High-volume sending strategy

How you send your emails is just as important as what you send. If you’re sending a large volume of emails, follow these steps to build and maintain a strong sender reputation.
1

Warm up your inboxes

Don’t go from zero to a thousand emails overnight. Email providers get suspicious of new inboxes that immediately send a high volume. Start slow and gradually increase your sending volume over several days or weeks. This “warm-up” process signals that you’re a legitimate sender.
Example warm-up schedule:
  • Day 1: 10 emails/inbox
  • Day 2: 20 emails/inbox
  • Day 3: 40 emails/inbox
  • …and so on, doubling until you reach your target.
See Rate limits for per-inbox sending limits.
2

Distribute across inboxes

Instead of sending 10,000 emails from a single inbox, send 100 emails from 100 different inboxes. This distributes your sending volume, reduces the risk of any single inbox getting flagged, and looks much more natural to email providers. OpenMail’s ability to create inboxes at scale makes this easy to implement.
3

Protect your reputation

A domain’s reputation is its most valuable asset. If you use a custom domain for outreach, consider rotating sending identities so a reputation hit on one doesn’t impact the rest.

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High-impact content strategy

The content of your email plays a huge role in whether it’s seen as a valuable message or as spam.
1

Personalize everything

Address your recipients by name in the subject line and email body. Use other data points you have to make the email feel like a one-to-one conversation, not a mass blast. Generic emails are a major red flag for spam filters.
2

Write like a human, not a marketer

Avoid “spammy” keywords (e.g., “free,” “buy now,” “limited time offer”), excessive exclamation points, and ALL CAPS. Write in a natural, conversational tone. The goal is to start a conversation, not to close a sale in the first email.
3

Skip images and tracking pixels

Images embedded in the email body set off spam filters. Open-tracking pixels are just tiny images encoded into the body — email providers know it. Drop them to improve deliverability.
4

Delay your links

Email providers are wary of links, especially in the first message of a conversation. Send your initial outreach with no links or images. Wait for the recipient to reply, and then send your call-to-action link. This looks like a natural conversation to providers.
5

Always include plain text

Email providers often flag HTML-only emails as spam. Including a plain text alternative demonstrates legitimacy and increases your chances of reaching the inbox.

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Suppressions

OpenMail automatically maintains a suppression list for bounces and spam complaints. Recipients who bounce or mark your email as spam are suppressed from future sends.

Suppressions

Learn how suppression lists work and how to manage them.

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What’s next

Rate limits

Per-inbox send limits and cold outreach throttling.

Idempotency

Safe retries so agents never send duplicate emails.

Inboxes

Address formats, external IDs, and multi-tenant routing.

API reference

Full endpoint documentation.