How to suggest email typo corrections?

I don't know anything doing this.

But as you have the name asked in the same form, you can try to search for it in the email address and compute the distance between substrings of the email address and the name.

If you find a word that is only one letter away, just warn the user about a possible typo.

But I would not block the form submission, just a small warning.

I put something quick and dirty together that seems to be doing the trick:

const splitCharsRegex = /[_. ,-]/;
const split = (str) => str.split(splitCharsRegex).map(s => s.toLowerCase());

// Source: https://gist.github.com/andrei-m/982927/0efdf215b00e5d34c90fdc354639f87ddc3bd0a5
const levenshteinDistance = function(a, b) {
	if(a.length == 0) return b.length;
	if(b.length == 0) return a.length;

	// swap to save some memory O(min(a,b)) instead of O(a)
	if(a.length > b.length) {
		var tmp = a;
		a = b;
		b = tmp;
	}

	var row = [];
	// init the row
	for(var i = 0; i <= a.length; i++){
		row[i] = i;
	}

	// fill in the rest
	for(var i = 1; i <= b.length; i++){
		var prev = i;
		for(var j = 1; j <= a.length; j++){
			var val;
			if(b.charAt(i-1) == a.charAt(j-1)){
				val = row[j-1]; // match
			} else {
				val = Math.min(row[j-1] + 1, // substitution
						prev + 1,     // insertion
						row[j] + 1);  // deletion
			}
			row[j - 1] = prev;
			prev = val;
		}
		row[a.length] = prev;
	}

	return row[a.length];
}

// Source: https://stackoverflow.com/questions/40654895/javascript-generating-all-permutations-of-an-array
const combo = (c) => {
	let r = [],
			len = c.length,
			tmp = [];

	function nodup() {
		var got = {};
		for (var l = 0; l < tmp.length; l++) {
			if (got[tmp[l]]) return false;
			got[tmp[l]] = true;
		}
		return true;
	}
	function iter(col) {
		let l, rr;
		if (col === len) {
			if (nodup()) {
				rr = [];
				for (l = 0; l < tmp.length; l++)
					rr.push(c[tmp[l]]);
				r.push(rr);
			}
		} else {
			for (l = 0; l < len; l ++) {
				tmp[col] = l;
				iter(col +1);
			}
		}
	}
	iter(0);
	return r;
}

const getSuggestion = (email = '', name = '') => {
	const username = email.split('@')[0];
    const emailWords = split(username) || [];
    const nameWords = split(name) || [];

    // Check complete username <> name, in different orders
	if (!username.match(splitCharsRegex)) {
		const allPossibleOrdersOfNameWords = combo(nameWords);
		for (let words of allPossibleOrdersOfNameWords) {
			if (levenshteinDistance(username.toLocaleLowerCase(), words.join('')) === 1) {
				return email.replace(username, words.join(''));
			}
		}
	}

    // Check each part of the username
    for (let emailWord of emailWords) {
        const nameWord = nameWords.find(w => levenshteinDistance(emailWord, w) === 1);
        if (nameWord) return email.replace(emailWord, nameWord);
    }
}

console.log(getSuggestion('john.dooe@gmail.com', 'John Doe')); // returns "john.doe@gmail.com"