This is anecdotal but here's the breakdown of top 10 e-mail providers from my database, does not look like a monopoly:

    MariaDB > SELECT SUBSTRING_INDEX(email, '@', -1) AS domain, COUNT(*) AS cnt FROM accounts GROUP BY domain HAVING domain != '' ORDER BY cnt DESC LIMIT 10;
    +-------------+-------+
    | domain      | cnt   |
    +-------------+-------+
    | hotmail.com | 38015 |
    | gmail.com   | 16280 |
    | yahoo.com   |  4080 |
    | o2.pl       |  2321 |
    | wp.pl       |  2206 |
    | live.com    |  1415 |
    | outlook.com |   814 |
    | interia.pl  |   609 |
    | hotmail.es  |   590 |
    | live.se     |   521 |
    +-------------+-------+
    10 rows in set (0.044 sec)