How do phishing detection systems flag legitimate senders?

You've done everything right. Your domain is authenticated, your list is clean, your content is helpful. And still, someone's security gateway pulls your email into quarantine because it "looks like phishing." That's a false positive, and it happens more than most senders realize.

Here's how phishing detection actually works. Modern filters don't just look for one bad signal. They run your email through a mix of rule-based checks, reputation scoring, and machine learning models trained on real phishing campaigns. The problem is those models learned from attackers who pretend to be legitimate. So when your legitimate email shares patterns with those attacks, the filter scores it poorly.

The most common triggers are worth knowing by name.

Brand name in content, unrelated sending domain. If your email mentions "your Microsoft account" or references "PayPal" while sending from captain@deepcurrent.io, filters notice the mismatch. Phishing emails do exactly this. They impersonate a brand in the copy while sending from a domain the brand doesn't own. Your email may be totally legitimate, but the pattern matches.

Authentication gaps. If your SPF, DKIM, or DMARC alignment isn't clean, filters treat that as a major red flag. A From address that doesn't match the signing domain looks like spoofing, even when it isn't. Phishers rely on exactly this kind of misalignment to hide behind borrowed trust.

Login links to unfamiliar domains. If your email includes a link to a sign-in page, security gateways follow that link. If the destination domain is young, unrecognized, or has low reputation, the filter may block the whole message. This catches legitimate SaaS senders regularly, especially when they use third-party login flows or custom subdomains that haven't built up any reputation yet.

Urgency language about accounts. Phrases like "verify your account now," "your access will be suspended," or "action required to avoid losing access" are classic phishing language. Filters have seen billions of these. If you're a real company sending a genuine account alert, you can still trigger those pattern matches.

New domain or cold sending. Phishing campaigns often use freshly registered domains. If you've just launched a domain and you're sending at volume before you've built any reputation, filters treat you with the same suspicion they'd apply to a domain registered yesterday for an attack.

The fix isn't to sanitize your language beyond recognition. It's to make your email structurally unlike phishing. Tight authentication (SPF, DKIM, DMARC all aligned), a sending domain with history, consistent branding, and links to domains you actually own. When those signals are clean, the filter has a harder time finding a reason to score you alongside the attackers.

And if you're hitting enterprise security gateways in particular (tools like Barracuda or security filters inside Microsoft 365), those have additional layers like sandboxing and link reputation checks. You can ask IT admins at recipient organizations to whitelist your domain, which bypasses most of this. But long term, your best protection is authentication that leaves no room for doubt.

You can check whether your SPF and DKIM are aligned correctly with our free tools, or if something's actively broken and you're not sure where to look, the SOS hotline is free.