Which Email Providers are the Most Popular?

At Mango's day job, he operates a mailing list for his customers. Approximately 2,000 customers have subscribed to it. He wanted to set up test email accounts with various webmail providers to be sure his emails arrived as intended. But which webmail providers should he choose? The most popular email providers were easily found with this MySQL query:

SELECT
SUBSTRING(`email`, LOCATE('@', `email`)) AS `domain`,
COUNT(*) AS `count`
FROM `table`
GROUP BY `domain`
ORDER BY `count` DESC

For our list, the results were: Major regional ISPs - 39% Hotmail - 14% Yahoo - 9% Gmail - 8% AOL - 1% The remaining 29% consisted of small ISPs and insignificant webmail providers, each less than 1% of the total.