You hit send on 50 emails yesterday. By this morning, you're bouncing 15 of them. By afternoon, you're in Spam on another 20. Your domain is flagged. Not blacklisted - not yet - but flagged. And you have no idea why.

The problem isn't usually one thing. It's almost always a combination of four specific technical failures that nobody tells you about until your domain is already damaged. I'm going to walk you through exactly what those are, how to check for each one, and what the actual fix looks like.

The Four Reasons Your Domain Gets Flagged

1. Your DNS Records Are Incomplete or Wrong

This is the foundation. If your DNS is broken, everything else fails - no matter what you do. Most people set up one record (usually SPF) and assume they're good. They're not.

You need four things:

If any of these are missing or malformed, you're already fighting uphill. The worst part? You might not notice for weeks. Your emails will send fine to Gmail and Outlook. They'll fail silently at corporate mail servers.

Here's what a correct SPF record looks like for a typical cold email setup:

v=spf1 include:sendgrid.net include:mandrillapp.com ~all

That "~all" at the end is important - it's a soft fail. If you use "~all" instead of "-all", mail servers will still accept emails from unauthorized IPs (bad for security, but good for deliverability while you're testing). Once you're confident, switch to "-all".

Your DMARC record should look like this to start:

v=DMARC1; p=none; rua=mailto:[email protected]

The "p=none" tells mail servers "don't reject anything yet, just report what happens." After a week, you'll see which emails are failing. Then you can move to "p=quarantine" (which sends failures to spam instead of inbox).

Check your records using a free tool like MXToolbox. If anything's missing or has a red X, that's your first problem.

2. Your Domain Has No Reputation (Or a Bad One)

A brand new domain with zero sending history is immediately suspicious. Mail servers assume new domains are used by spammers because, statistically, they often are.

The fix is warming. Not warmup tools - actual warming. You need to send real emails from your domain to real mailboxes for 7-10 days before you start your real campaign.

Here's the specific pattern that works: send 10 emails on day one. 20 on day two. 40 on day three. Keep doubling every other day until you hit 200-300 per day. Then hold there for 3 days before you launch your campaign.

Those emails need to be to real addresses that will actually engage (opens and replies help your reputation). They can't be test accounts. They can't bounce. If you're using lemlist or Mailchimp's contacts, it doesn't count - you need real, external mailboxes.

We have a detailed guide on how to warm up your email domain properly if you need the exact steps.

3. Your Sending Volume Is Growing Too Fast

This one catches people off guard. You did everything right - DNS is set up, domain is warmed. Then you get excited and send 500 emails in one day. Your domain gets flagged immediately.

Mail servers track velocity. They expect a domain to send roughly the same amount every day. A sudden jump from 300/day to 500/day looks like an account compromise or a spam botnet hijack.

The fix: grow your sends by no more than 20-30% per day. If you're sending 300 emails today, send 360 tomorrow. Not 450. The extra 60-90 emails might seem like nothing, but to mail servers, it's the difference between "normal business growth" and "something is wrong."

4. You're Sending From a Shared IP or Cheap Provider

If you're using a Gmail account, a free SendGrid tier, or a hosting provider's built-in mail, you're sharing an IP with thousands of other people. One of them could be running a spam operation. One bad neighbor tanks your reputation.

A dedicated IP isn't always necessary, but it does matter if you're serious about deliverability. If you can't afford a dedicated IP, at least use a provider (like SendGrid Pro or Mailgun) where you get some isolation from other users.

The cost difference is minimal - maybe $30-50/month extra. If cold email is actually driving your business, it's worth it.

How to Check If Your Domain Is Actually Flagged

Don't guess. Check. Use these tools:

Run a test email through Mail-tester. You should score at least 8/10. If you're below 7, something is wrong - and it's usually one of the four issues above.

The Recovery Plan (If You're Already Flagged)

If your domain is already flagged, stopping everything and fixing it is faster than trying to power through.

Here's the sequence: First, fix your DNS records. Second, stop sending for 3 days - let your domain's reputation reset slightly. Third, warm up again using the pattern above. Fourth, start your campaign again, but at half the volume you were originally planning.

Most domains recover in 2-3 weeks. Some take longer if they hit a blacklist - we have a guide on that specific situation if you need it.

The goal isn't to get flagged in the first place. Fix these four things before you send anything, and you won't have to recover.

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