How does a domain get flagged as suspicious?

You send a campaign and open rates crater. Replies dry up. A contact messages you to say your emails keep hitting their spam folder. Sound familiar? Your domain may be getting flagged as suspicious, and the tricky part is that it rarely happens all at once. It builds up quietly.

Here are the main ways a domain earns that status, plus what to actually check.

Authentication failures. If your SPF, DKIM, or DMARC records are broken, missing, or misaligned, mailbox providers take notice fast. Legitimate senders keep their authentication in order. When yours isn't, it looks like someone is spoofing your domain or that you just don't know what you're doing. Either way, that's a red flag. Check your SPF with our free SPF checker, and verify your DKIM is signing correctly before anything else.

Spamtrap hits. Spamtraps are addresses that no real person uses. Hitting one tells mailbox providers and blocklists that your list contains addresses it shouldn't. This usually means you're sending to old, purchased, or poorly collected contacts. Even a small number of spamtrap hits can do serious damage to how your domain is perceived.

High complaint rates. When recipients click "this is spam" on your emails, that signal goes straight back to the mailbox provider. A complaint rate above 0.1% at Gmail will start hurting your reputation. Above 0.3% and you're in real trouble. This is one of the clearest signals that people don't want what you're sending.

Blocklist appearances. If your domain or sending IP lands on a major blocklist like Spamhaus, Barracuda, or Spamcop, receiving mail servers will often block or filter your emails automatically. You can check whether your domain is listed right now with our free blocklist checker.

Behavioral patterns that match spam profiles. A brand-new domain sending thousands of emails in its first week looks suspicious. So does a sudden volume spike from a domain that normally sends modestly. Mailbox providers watch volume patterns over time, and anything that breaks the expected pattern gets extra scrutiny.

Infrastructure associations. If your domain shares sending IPs with other flagged domains, or your emails link to domains that are already on blocklists, that guilt by association is real. Filters don't just look at your domain in isolation.

The honest diagnostic checklist if you think you're flagged:

• Run your domain through a blocklist checker (links above)
• Check your SPF, DKIM, and DMARC are all passing
• Pull your Google Postmaster Tools data if you're sending to Gmail users (complaint rates, domain reputation score)
• Look at your bounce rates from recent campaigns, especially hard bounces
• Review when you last cleaned your list

Suspicion builds gradually, but it can also move fast once it tips. If you're seeing signs right now and want a second set of eyes, our SOS hotline is free and we actually pick up.