Nothing can tell you an address exists, without sending to it
The technique verification services use is to open a conversation with the receiving mail server and ask about the address without delivering anything. It works often enough to be commercially useful, and it fails in both directions: many providers accept every address regardless, so a bad one comes back valid, and some refuse the conversation entirely or answer differently depending on who is asking, so a perfectly good address comes back doubtful.
It also has costs. Probing at volume looks like the reconnaissance that precedes a spam run, and the reputation of the address doing the probing suffers accordingly. That is one reason it is a service rather than a feature - the risk is absorbed by infrastructure built for it. A tool that promises certainty here is overstating what the protocol permits anyone to know.
What can be established without contacting anyone
Quite a lot, and it catches most real mistakes. Whether the syntax is valid. Whether the domain resolves. Whether it publishes MX records, without which it cannot receive mail at all - which immediately catches typos in the domain, expired domains, and addresses at a hostname that was never a mail domain. Whether the provider is a disposable one.
On a signup form that set is the useful one, because the errors it finds are the errors people make: the mistyped domain, the address at a company that no longer exists, the throwaway inbox. Those checks are also instant and involve no third party learning your address list.
Disposable addresses are a decision, not an error
A throwaway address is valid, deliverable, and intended to stop working. Someone using one is telling you they want the thing behind the signup and not the relationship, which is information rather than fraud. Blocking them outright is a legitimate choice for a trial that costs you money, and the wrong choice for a newsletter where the cost is nothing.
The useful posture is usually to treat it as a signal rather than a gate: allow the signup, do not count it as a lead, and do not let it inflate the numbers a decision will later be made from.