пример 1
[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?

пример 2
([a-zA-Z0-9]+[a-zA-Z0-9._-]*@[a-zA-Z0-9_-]+(\.[a-zA-Z0-9]+[a-zA-Z0-9_-]*)+)

пример 3
[A-Za-z0-9._-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}

az09 = list('abcdefghijklmnopqrstuvwxyz1234567890.-@')

import dns.resolver
def check_mx_domain(email):
    try:
        domain = email.split('@')[1]
    except IndexError:
        return False
    if not domain:
        return False
    try:
        answers = dns.resolver.query(domain, 'MX')
        for data in answers:
            if data.exchange:
                return True
    except (dns.resolver.NXDOMAIN, dns.resolver.Timeout, dns.exception.DNSException):
        return False
    return False