There are many tools that will check to see if the domain is valid and if so, it will often assume the email address must be real.
It's kind of like not knowing if someone is home. You can call, but maybe they are napping or in the yard. The only way to really know is to go to their house. With email, the only way (I have found) to know if the email is real is to send a message.
Many setup additional hosts for this purpose. Send an email without any link or sales text. Take the bad ones, save them to your bounce list and move the good list to your good account.
IF YOU DO NOT DO THIS, you will not only burn up your IP on 1and1, but you will also make every domain and email address you use become a red flag for the ISPs. Do not make that mistake.