You know the drill. You take your contact list, run it through ZeroBounce, NeverBounce or a similar service, get back a lovely 95 percent “valid”, and launch the campaign. And a pile of emails still comes back as undeliverable. How is that possible when you verified the list?
The answer is uncomfortable. These services do not actually verify most of the addresses for real.
What these services really do
There is a simple test that gives it away. Send 10,000 emails for verification and you get the result back in a few minutes. Sounds great, but that speed is exactly the problem. Real mailbox verification over SMTP cannot be done that fast.
Why? To truly find out whether a mailbox exists, you have to connect to the recipient’s mail server and ask it about that specific address. But large providers like Gmail and Microsoft 365 limit how many such queries they accept from one address. Many servers also use something called greylisting. On the first attempt they reply with a temporary rejection, and you have to wait and try again, sometimes minutes or hours later. Honest verification of a large list therefore takes hours, not minutes.
So when you get it back in five minutes, it means only one thing. Most of the addresses never asked any mail server at all. They were simply compared against the service’s internal database and run through its heuristics and AI models. Have we seen this address before? Is the domain known as good or bad? Does the address look “risky”? That is a guess, not verification.
Why it fails
A guess works right up until it is wrong. And because no database covers the whole world, and people keep closing and opening mailboxes, non-existent addresses that looked fine will always slip through.
Catch-all domains are the worst. These are servers set up to accept email at any address, even a completely made-up one. The verification service has no way to tell whether a specific mailbox exists there, so it often marks them as “valid” simply because the domain has delivered in the past. You then send to addresses that nobody actually reads, or that do not exist at all.
The second problem comes after you hit send
And this is where it really hurts. Every email that bounces back as undeliverable is a black mark against your domain. Mailbox providers count your bounces. A high number of rejected messages is a clear signal to them that you are sending to unverified lists, and that is exactly how spammers behave.
A few percent of bounces is enough to start ruining your domain reputation. In the worst case you land on a blacklist such as Spamhaus, and your emails stop reaching inboxes across the board. And it is not just campaigns. Your invoices and everyday communication to your own customers stop getting through too, until you clean the domain up. That can take weeks.
So the verification that was supposed to protect you has handed you a problem instead.
The only real verification is over SMTP
There is only one approach that works. Actually connecting to the recipient’s mail server and having it confirm whether the specific mailbox exists. Yes, it is slower. You have to handle greylisting, work around the limits of the big providers, and probe from clean, trusted infrastructure so the server even responds. But it is the only path that gives a real answer.
Real SMTP verification from good infrastructure is the only thing that can tell how a catch-all domain behaves, correctly query mailboxes on Gmail and Microsoft 365, and reveal addresses on servers with a troubled reputation. Where a database service just guesses “valid”, real verification tells the truth.
What this means for you
If you care about deliverability and about your domain reputation, fast database verification is a trap. It looks like you have a clean list, but you are still sitting on non-existent mailboxes that will wreck your domain on the very first campaign.
We verify addresses for real, over SMTP, from our own clean infrastructure. It takes longer, but the result is a list you can send to without risking your own domain. And with a managed campaign it is included in the price.