How do I filter users based on email address?

I wasn’t sure if you meant users or commentators, so I took both.

SQL

The email is saved in $wpdb->comments in the column comment_author_email for comments.
It is saved in $wpdb->users in the column user_email for users.

To get the domain part from the email in comments, we can use:

SELECT (
    SUBSTR(
        `comment_author_email`,
        INSTR( `comment_author_email`, '@' ) + 1,
        LENGTH( `comment_author_email` ) - INSTR( `comment_author_email`, '@' )
    )
) AS domain

A complete SQL query to get the most used domains from WordPress’ comments table could look like this:

SELECT (
    SUBSTR(
        `comment_author_email`,
        INSTR( `comment_author_email`, '@' ) + 1,
        LENGTH( `comment_author_email` ) - INSTR( `comment_author_email`, '@' )
    )
) AS domain,
COUNT(*) AS amount
FROM        `$wpdb->comments`
WHERE       `comment_approved` = 'spam'
GROUP BY    domain
HAVING      amount > 1
ORDER BY    amount DESC
LIMIT 0, 150

Dashboard Widget

I have used that query to create a simple dashboard widget.

Download on GitHub.

<?php # -*- coding: utf-8 -*-
namespace WPSE;
/**
 * Plugin Name: Top Mail Domains
 * Description: List the most used domain names from spammers and users in two dashboard widgets.
 * Version:     2013.06.02
 * Author:      Thomas Scholz
 * Author URI:  http://wpkrauts.com
 * License:     MIT
 */

\is_admin() && \add_action(
    'wp_dashboard_setup',
    array (
        Top_Spam_Mail_Domains::get_instance(),
        'setup'
    )
);

class Top_Spam_Mail_Domains
{
    protected static $instance;

    public static function get_instance()
    {
        if ( NULL === self::$instance )
            self::$instance = new self;

        return self::$instance;
    }

    public function setup()
    {
        $comment_model  = new Comment_Data;
        $comment_widget = new Widget( $comment_model );

        \wp_add_dashboard_widget(
            'top_comment_spam_domains',
            'Top Comment Mail Domains',
            array ( $comment_widget, 'display' )
        );

        $user_model  = new User_Data;
        $user_widget = new Widget( $user_model );

        \wp_add_dashboard_widget(
            'top_user_spam_domains',
            'Top User Mail Domains',
            array ( $user_widget, 'display' )
        );
    }
}

class Widget
{
    protected $source;

    public function __construct( Data $source )
    {
        $this->source = $source;
    }

    public function display()
    {
        $data = $this->source->get_domains(1);

        if ( empty ( $data ) )
            return print 'No domains found.';

        print $this->data_to_table( $data );
    }

    protected function data_to_table( Array $data )
    {
        $table="<table class="widefat">";
        $header="<tr><th>Domain</th><th>Amount</th></tr>";
        $table .= "<thead>$header</thead><tfoot>$header</tfoot>";

        foreach ( $data as $result )
            $table .= sprintf(
                '<tr class="%1$s"><td>%2$s</td><td>%3$s</td></tr>',
                $this->row_class(),
                \esc_html( $result->domain ),
                \esc_html( $result->amount )
            );

        return "$table</table>";
    }

    protected function row_class()
    {
        static $count = 1;

        return ( $count++ % 2 ) ? '' : 'alt';
    }
}

abstract class Data
{
    public function get_domains( $min = 5 )
    {
        global $wpdb;

        $sql = $this->get_sql( absint( $min ) );
        return $wpdb->get_results( $sql );
    }

    abstract protected function get_sql( $min );
}

class Comment_Data extends Data
{
    protected function get_sql( $min )
    {
        global $wpdb;

        return "
SELECT (
    SUBSTR(
        `comment_author_email`,
        INSTR( `comment_author_email`, '@' ) + 1,
        LENGTH( `comment_author_email` ) - INSTR( `comment_author_email`, '@' )
    )
) AS domain,
COUNT(*) AS amount
FROM        `$wpdb->comments`
WHERE       `comment_approved` = 'spam'
GROUP BY    domain
HAVING      amount > $min
ORDER BY    amount DESC
LIMIT 0, 150
        ";
    }
}


class User_Data extends Data
{
    protected function get_sql( $min )
    {
        global $wpdb;

        return "
        SELECT (
        SUBSTR(
        `user_email`,
        INSTR( `user_email`, '@' ) + 1,
        LENGTH( `user_email` ) - INSTR( `user_email`, '@' )
        )
        ) AS domain,
        COUNT(*) AS amount
        FROM        `$wpdb->users`
        GROUP BY    domain
        HAVING      amount > $min
        ORDER BY    amount DESC
        LIMIT 0, 150
        ";
    }
}