Array
(
    [aaa@mail.com] => ccc@mail.com
    [bbb@mail.com] => ddd@mail.com
    [ccc@mail.com] => aaa@mail.com
    [ddd@mail.com] => bbb@mail.com
)

<?php namespace ironwoodsphp_classes;

final class PairSelector
{
    private $assigned_pairs;
    private $emails;
    private $total_emails = 0;

    private $err_no_emails = 'No emails registered or less of two!';

    /**
     * Gets pairs of emails in an associative array
     *
     * @param  array        $arr_emails
     * @return array
     */
    public function getPairs(array $arr_emails)
    {
        $this->emails = $arr_emails;

        /**
         * Is necesary setted at least two emails to use this class
         *
         */
        if (! $this->searchForMinimalCondicion()) {
            die('ERR -> ' . $this->err_no_emails);
        }

        $total_emails = count($arr_emails);
        $this->total_emails = $total_emails;

        $i = 0;
        $this->assigned_pairs = array();
        while ($i < $total_emails) {

            $pos = $this->getPosition($i); //for email 2

            $email_1 = $arr_emails[$i];     //Take each email of array one time
            $email_2 = $arr_emails[$pos];   //Take each email of array one time
            $this->assigned_pairs[$email_1] = $email_2;

            $i++;
        }

        return $this->assigned_pairs;
    }

    /**
     * Gets a value from the array different to actual and unassigned before
     *
     * @param  int      $actual_pos
     * @return int
     */
    private function getPosition(int $actual_pos)
    {
        $pos = $actual_pos;
        $max = $this->total_emails - 1;
        while ($pos === $actual_pos || $this->isAssigned($pos)) {

            $pos = rand(0, $max);
        }

        return $pos;
    }

    /**
     * Searches an email between the destinataries already assigned
     *
     * @param  int      $pos
     * @return boolean
     */
    private function isAssigned(int $pos)
    {
        $email = $this->emails[$pos];
        foreach ($this->assigned_pairs as $destinatary) {

            if ($email === $destinatary) {
                return true;
            }
        }

        return false;
    }

    /**
     * Searches if the number of emails are at least two
     *
     * @return boolean
     */
    private function searchForMinimalCondicion()
    {
        return ($this->emails && (count($this->emails) > 1));
    }
} //class

<?php namespace ironwoodsphp_classestests;

require '../pairselector.php';
use ironwoodsphp_classesPairSelector;


$arr_emails = [
    'aaa@mail.com',
    'bbb@mail.com',
    'ccc@mail.com',
    'ddd@mail.com',
];

$ps = new PairSelector();
$res = $ps->getPairs($arr_emails);

echo '<pre>';
print_r($res);

echo '<hr>';

$res = $ps->getPairs(['usu@kas.es']);
print_r($res);

<?php namespace ironwoodsphp_classestests;
/*
cd c:/xampp/htdocs/php__classes/tests
phpunit PairSelectorTest.php --colors=always --repeat 10
*/
use PHPUnitFrameworkTestCase;
use ironwoodsphp_classesPairSelector;

$_BASE_PATH = dirname( __FILE__, 2 ) . '/'; //Only PHP 7
require $_BASE_PATH . 'pairselector.php';


class PairSelectorTest extends TestCase
{
    private $emails = [
        'aaa@mail.com',
        'bbb@mail.com',
        'ccc@mail.com',
        'ddd@mail.com',
    ];


    /**
     * @coversDefaultClass ironwoodsphp_classes
     * @covers PairSelector::getPairs
     */
    public function testIfReturnPairs()
    {
        $arr_pairs = $this->getPairs();

        self::assertTrue(is_array($arr_pairs)); //we have an array
        self::assertFalse(empty($arr_pairs));   //our array has elements

        return $arr_pairs;
    }

    /**
     * @coversDefaultClass ironwoodsphp_classes
     * @covers PairSelector::getPairs
     * @depends testIfReturnPairs
     */
    public function testIfReturnEqualNumberOfPairsThatEmailsInOrigin(
        Array $arr_pairs
    )
    {
        self::assertEquals(
            count($arr_pairs),
            count($this->emails)
        );

        return $arr_pairs;
    }

    /**
     * @coversDefaultClass ironwoodsphp_classes
     * @covers PairSelector::getPairs
     * @depends testIfReturnEqualNumberOfPairsThatEmailsInOrigin
     */
    public function testIfReturnAsKeysSameEmailsThatEmailsInOrigin(
        Array $arr_pairs
    )
    {
        $i = 0;
        foreach ($arr_pairs as $key => $value) {

            $email = $this->emails[$i];
            $msg = 'Email from pairs-key and origin-emails are different';
            self::assertEquals( $email, $key, $msg );

            $i++;
        }

        return $arr_pairs;
    }

    /**
     * @coversDefaultClass ironwoodsphp_classes
     * @covers PairSelector::getPairs
     * @depends testIfReturnAsKeysSameEmailsThatEmailsInOrigin
     */
    public function testIfKeysAndValuesAreDifferentsInArrayOfPairs(
        Array $arr_pairs
    )
    {
        foreach ($arr_pairs as $key => $value) {

            $msg = 'Email in key is the same that email in value';
            self::assertNotEquals($key, $value, $msg);
        }

        return $arr_pairs;
    }

    /**
     * @coversDefaultClass ironwoodsphp_classes
     * @covers PairSelector::getPairs
     * @depends testIfKeysAndValuesAreDifferentsInArrayOfPairs
     */
    public function testIfReturnArrayValuesAreAllDifferent(
        Array $arr_pairs
    )
    {
        $arr_pairs = array_unique($arr_pairs); //Filters duplicate elements
        $msg = 'Values in array of pairs have duplicate mails';
        self::assertEquals(
            count($this->emails),
            count($arr_pairs),
            $msg
        );

        return $arr_pairs;
    }

    /**
     * Runs "PairSelector->getPairs()" to get the pairs of emails
     *
     * @return      array
     */
    private function getPairs()
    {
        $ps = new PairSelector();

        return $ps->getPairs($this->emails);
    }
} //class