Invalid mailboxes, typos, duplicates, malformed addresses, non-existing domains, invalid MX-records (i.e. no email server found on this domain) and disposable mailboxes like "mailinator" etc.
Also, common gibberish and keyboard mashing like "asd", "asdf", "qweqweqwe" etc.
Email cleaners like this are either paid or free but limited to, like, 100 email addresses... This tool is unlimited.
We do have a technical limit of 20000 (twenty thousand) emails just to keep things from overheating, but hey, if your email list is over 20k you probably use an email marketing platform that already takes care of this.
We do not collect any addresses. In fact, most of it works inside your browser, the email addresses are not even sent to our servers at all.
We are Alex and Max, the makers of the awesome email ticketing system used by thousands of companies. Give it a try if you need a ticket management app for your IT team.
You need to maintain good sender reputation, otherwise your emails will start bouncing back and being falsely marked as "spam" by email services on the receiving end. A single malformed address can negatively impact deliverability of your "good" emails too, your whole email marketing campaign is on the line. A low quality list can also distort your metrics, conversion and open rates.