Check a file of email addresses against the WP User database to see if they're in use or not. This is not part of a command; it can be added to any.

user_recon.php

<?php

	/**
	 * Check a file of email addresses against the user database to see if those
	 * email addresses are currently in use.
	 *
	 * ## OPTIONS
	 *
	 * <file>
	 * : The file of users to import.
	 *
	 * ## EXAMPLES
	 *
	 *     wp COMMAND user_recon emails.csv
	 *
	 *     Sample emails.csv file:
	 *
	 *     email
	 *     [email protected]
	 *     [email protected]
	 *
	 * @synopsis <file>
	 */
	function user_recon( $args, $assoc_args ) {
		$filename = $args[0];

		if ( ! file_exists( $filename ) ) {
			WP_CLI::warning( sprintf( "Missing file: %s", $filename ) );
		}

		$total = $exists = $missing = 0;
		foreach ( new \WP_CLI\Iterators\CSV( $filename ) as $user ) {
			$total++;
			$email = sanitize_email( $user['email'] );

			if ( empty( $email ) ) {
				WP_CLI::warning( "`{$user['email']}` is not a valid email address." );
				continue;
			}

			$existing_user = get_user_by( 'email', $email );

			if ( $existing_user ) {
				$exists++;
				WP_CLI::line( "{$email} is currently in use by {$existing_user->user_login}" );
			} else {
				$missing++;
			}
		}

		WP_CLI::success( "Process complete! Found {$exists} email addresses in use and {$missing} not in use across {$total} entries in {$filename}." );
	}