imap_open : couldn't open stream to my mail server

Thank you for your answers. My mistake was not coming from the connection but rather a loop that crashed the server when I had too much email :

imap_open("{mail.domain.com:143/novalidate-cert}INBOX", 'login', 'password')

$mails = imap_search($stream, 'UNSEEN');

rsort($mails);
foreach ($mails as $mailId) {
  imap_fetch_overview($stream, $mailId, 0);
} //that was the mistake when email number is too big!