Should you block temp mail at sign-up?

Short answer: flag it, don't just block it. Block only where a fake account costs you something real.

Why sites block temp mail

Free trials and bonuses. A new address means a new trial. One person can take dozens.
Fake accounts in bulk. Bots sign up with temp mail because it needs no account of its own.
You can't reach the person later. The inbox is gone in an hour, so password resets and receipts go nowhere.

Why blocking can hurt

Careful people use it too. Many just want to try your product without spam.
Lists make mistakes. Some lists name privacy forwarding services (DuckDuckGo, Firefox Relay, SimpleLogin) or whole free DNS services. Blocking those turns away honest users.
It is easy to get around. Real Gmail accounts, the Gmail dot trick and private domains pass every list.

A sensible approach

On “disposable”: ask for an address they keep, and explain why in one sentence (“we send your receipt and password resets there”).
On “suspicious”: let them in, but hold back the costly part (a trial, credits, a payout) until they confirm the address.
On “undeliverable”: point out the typo. The domain cannot receive mail.
Always: store the cleaned-up Gmail address (dots and + part removed) next to the one they typed, and allow one free trial per real inbox.

const r = await fetch("https://email.tooljar.dev/api/check?q=" + encodeURIComponent(email)).then((x) => x.json());

if (r.verdict === "disposable") return askForAnotherAddress();
if (r.verdict === "undeliverable") return showTypoHint();
const key = r.normalized ?? email.toLowerCase(); // one trial per real inbox
const trialAllowed = r.verdict === "ok" && !(await trialUsed(key));

Questions

Is it legal to block temp mail addresses?

Generally yes: you decide who can sign up for your service. If you block, say so plainly on the form, so honest people know what to do.

Will blocking temp mail stop fake accounts?

It stops the lazy ones. Determined abusers use real Gmail accounts, the Gmail dot trick or private domains. Combine the check with rate limits, the Gmail clean-up and a check before anything costly, like a free trial.

Should I block DuckDuckGo, Firefox Relay or SimpleLogin addresses?

We suggest not. They forward to the person's real inbox and last as long as the person wants. The people who use them are usually careful about privacy, not abusers.

Try any address on the temp mail checker, or read how it works.