SPF, DKIM, and DMARC
Set these up before your first send. They are DNS records that prove to receiving providers you are who you say you are. All inboxes useomail.sh by default. For production outreach at scale, send from a verified custom domain like outreach@yourcompany.com so your reputation belongs to you.
Inboxes
Learn about inbox address formats and how to configure your sending domain.
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.See Rate limits for per-inbox sending limits.
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.
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.
Reputation, not quota. Inbox distribution protects per-inbox
reputation and helps you stay under per-inbox daily limits. Your
account cold cap is shared across all inboxes —
adding more inboxes does not multiply total cold volume. Upgrade your plan
if you need a higher ceiling.
3
Protect your reputation
A domain’s reputation is slow to build and quick to burn. If you use a custom
domain for outreach, consider rotating sending identities so a reputation
hit on one doesn’t impact the rest.
High-impact content strategy
Content decides whether a spam filter reads your email as a message or as junk.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.
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.
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.