before you press send
Some of your list was never going to arrive.
Paste it in. We will tell you which addresses sit on domains that accept no mail, which are one character away from a real inbox, and which sit on catch-all domains where no tool on earth can tell you anything. Takes a few seconds and nothing is stored.
Free, no signup, up to 500 addresses a go. Nothing you paste is stored.
The part we cannot do from here
Confirming that a specific mailbox exists means opening an SMTP conversation with the receiving server on port 25. Every major cloud provider blocks outbound port 25 to suppress spam, so no hosted tool can do it, ours included. Anything that claims to is inferring and calling it verification.
So the real gate runs on your machine, where the probe works and your list never goes anywhere. The most honest way to test it is against a campaign you have already sent, because you already know which addresses bounced:
git clone https://github.com/omm9846/verdict.git cd verdict/backend && pip install -r requirements.txt python -m engine backtest bounced.csv
It reports how many of your bounces were knowable before you sent, and how many were not. The second number is the one nobody else will show you.