Every email you send to a non-existent or invalid address is a hard bounce. Bounces are the #1 reputation destroyer in cold email. Email verification is the prerequisite to every send, and the step most teams skip when they're in a rush.
Checks whether an email address actually exists and will accept mail before you try to send. Methods:
Output per address:
Hard bounce rates damage sender reputation badly. Rule of thumb:
A single unverified list can push you from 1% to 6% bounce rate overnight. One campaign can damage infrastructure for weeks.
Apollo, Clay, and some email finders include verification. Usually less rigorous than standalone verifiers. Good as first pass, but double-check with standalone for important campaigns.
Instantly, Smartlead have built-in verification. Convenient, quality varies.
Some domains accept all email regardless of whether the specific address exists. Verifiers can't tell which specific addresses at catch-alls are real.
Options:
Pragmatic: include catch-alls for large corporate domains (likely real if pattern matches company convention), exclude for unknown small-domain catch-alls.
At scale (100K+ addresses/month), sub-$100/month easily. Cheap insurance against reputation damage.
Same person may appear multiple times across exports. Dedupe by email.
Current customers, existing relationships, past positive replies should not be in cold lists.
Global DNC list applied before every campaign send.
Don't re-bounce. If an address bounced once, it stays invalid until you have reason to think it changed.
info@, sales@, contact@, these get more spam complaints than personal addresses. Include only if specifically targeting that role.
If the title doesn't match your ICP, remove. Sending to "intern" or "analyst" when your ICP is "VP" wastes volume.
Addresses verified 6 months ago may now be invalid. People change jobs. Domains close. Re-verify high-value lists quarterly or before major campaigns.
Never upload a list to a sending tool without running verification first. Even lists from Apollo or ZoomInfo. Even "verified" lists from Clay. Fresh verification before every campaign is cheap and protects the infrastructure.
Watch bounce rate per campaign in the sending tool. Rules:
Don't continue campaigns with high bounce rates hoping it improves. It won't, and the damage compounds.
If a data source consistently produces high bounce rates even after verification, that source is unreliable. Switch or stop using it.
Over time you build intuition: "Apollo accuracy for X segment is poor," "Clay+LinkedIn for Y segment is excellent." Use the right source for each segment.
Next: Intent signals.